diff --git a/src/libraries/System.Linq/src/System.Linq.csproj b/src/libraries/System.Linq/src/System.Linq.csproj
index 68b88631587ace..60d926e02917ad 100644
--- a/src/libraries/System.Linq/src/System.Linq.csproj
+++ b/src/libraries/System.Linq/src/System.Linq.csproj
@@ -24,6 +24,7 @@
     <Compile Include="System\Linq\DefaultIfEmpty.SpeedOpt.cs" />
     <Compile Include="System\Linq\Distinct.SpeedOpt.cs" />
     <Compile Include="System\Linq\Grouping.SpeedOpt.cs" />
+    <Compile Include="System\Linq\Index.SpeedOpt.cs" />
     <Compile Include="System\Linq\Iterator.SpeedOpt.cs" />
     <Compile Include="System\Linq\Lookup.SpeedOpt.cs" />
     <Compile Include="System\Linq\OfType.SpeedOpt.cs" />
diff --git a/src/libraries/System.Linq/src/System/Linq/Index.SpeedOpt.cs b/src/libraries/System.Linq/src/System/Linq/Index.SpeedOpt.cs
new file mode 100644
index 00000000000000..8393e0c3d6a258
--- /dev/null
+++ b/src/libraries/System.Linq/src/System/Linq/Index.SpeedOpt.cs
@@ -0,0 +1,213 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using System.Diagnostics;
+using static System.Linq.Utilities;
+
+namespace System.Linq
+{
+    public static partial class Enumerable
+    {
+        private sealed partial class IEnumerableIndexIterator<TSource> : Iterator<(int Index, TSource Item)>
+        {
+            private readonly IEnumerable<TSource> _source;
+            private int _index;
+            private IEnumerator<TSource>? _enumerator;
+
+            public IEnumerableIndexIterator(IEnumerable<TSource> source)
+            {
+                Debug.Assert(source is not null);
+                _source = source;
+            }
+
+            private protected override Iterator<(int Index, TSource Item)> Clone() =>
+                new IEnumerableIndexIterator<TSource>(_source);
+
+            public override void Dispose()
+            {
+                if (_enumerator is not null)
+                {
+                    _enumerator.Dispose();
+                    _enumerator = null;
+                }
+
+                base.Dispose();
+            }
+
+            public override bool MoveNext()
+            {
+                switch (_state)
+                {
+                    case 1:
+                        _enumerator = _source.GetEnumerator();
+                        _index = -1;
+                        _state = 2;
+                        goto case 2;
+                    case 2:
+                        Debug.Assert(_enumerator is not null);
+
+                        if (_enumerator.MoveNext())
+                        {
+                            _current = (checked(++_index), _enumerator.Current);
+                            return true;
+                        }
+
+                        Dispose();
+                        break;
+                }
+
+                return false;
+            }
+
+            public override IEnumerable<TResult2> Select<TResult2>(Func<(int Index, TSource Item), TResult2> selector) =>
+                new IEnumerableSelect2Iterator<TSource, TResult2>(_source, CombineSelectors((TSource x, int i) => (i, x), selector));
+
+            public override (int Index, TSource Item)[] ToArray()
+            {
+                int index = -1;
+
+                if (_source.TryGetNonEnumeratedCount(out int known))
+                {
+                    var array = new (int Index, TSource Item)[known];
+
+                    foreach (TSource item in _source)
+                    {
+                        array[checked(++index)] = (index, item);
+                    }
+
+                    return array;
+                }
+
+                SegmentedArrayBuilder<(int Index, TSource Item)>.ScratchBuffer scratch = default;
+                SegmentedArrayBuilder<(int Index, TSource Item)> builder = new(scratch);
+
+                foreach (TSource item in _source)
+                {
+                    builder.Add((checked(++index), item));
+                }
+
+                (int Index, TSource Item)[] result = builder.ToArray();
+                builder.Dispose();
+                return result;
+            }
+
+            public override List<(int Index, TSource Item)> ToList()
+            {
+                List<(int Index, TSource Item)> list = _source.TryGetNonEnumeratedCount(out int known) ? new(known) : [];
+                int index = -1;
+
+                foreach (TSource item in _source)
+                {
+                    list.Add((checked(++index), item));
+                }
+
+                return list;
+            }
+
+            public override int GetCount(bool onlyIfCheap)
+            {
+                // In case someone uses Count() to force evaluation of
+                // the selector, run it provided `onlyIfCheap` is false.
+                if (onlyIfCheap)
+                {
+                    return _source.TryGetNonEnumeratedCount(out int known) ? known : -1;
+                }
+
+                int count = 0;
+
+                foreach (TSource item in _source)
+                {
+                    checked
+                    {
+                        count++;
+                    }
+                }
+
+                return count;
+            }
+
+            public override (int Index, TSource Item) TryGetElementAt(int index, out bool found)
+            {
+                if (_source is Iterator<TSource> iterator)
+                {
+                    return iterator.TryGetElementAt(index, out found) is var element && found ? (0, element!) : default;
+                }
+
+                if (index >= 0)
+                {
+                    IEnumerator<TSource> e = _source.GetEnumerator();
+                    int enumeratorIndex = -1;
+
+                    try
+                    {
+                        while (e.MoveNext())
+                        {
+                            if (index == 0)
+                            {
+                                found = true;
+                                return (checked(++enumeratorIndex), e.Current);
+                            }
+
+                            index--;
+                        }
+                    }
+                    finally
+                    {
+                        (e as IDisposable)?.Dispose();
+                    }
+                }
+
+                found = false;
+                return default;
+            }
+
+            public override (int Index, TSource Item) TryGetFirst(out bool found)
+            {
+                if (_source is Iterator<TSource> iterator)
+                {
+                    return iterator.TryGetFirst(out found) is var first && found ? (0, first!) : default;
+                }
+
+                using IEnumerator<TSource> e = _source.GetEnumerator();
+
+                if (e.MoveNext())
+                {
+                    found = true;
+                    return (0, e.Current);
+                }
+
+                found = false;
+                return default;
+            }
+
+            public override (int Index, TSource Item) TryGetLast(out bool found)
+            {
+                if (_source is Iterator<TSource> iterator && iterator.GetCount(true) is not -1 and var count)
+                {
+                    return iterator.TryGetLast(out found) is var last && found ? (count - 1, last!) : default;
+                }
+
+                using IEnumerator<TSource> e = _source.GetEnumerator();
+
+                if (e.MoveNext())
+                {
+                    found = true;
+                    TSource lastElement = e.Current;
+                    int lastIndex = -1;
+
+                    while (e.MoveNext())
+                    {
+                        lastElement = e.Current;
+                        lastIndex++;
+                    }
+
+                    return (lastIndex, lastElement);
+                }
+
+                found = false;
+                return default;
+            }
+        }
+    }
+}
diff --git a/src/libraries/System.Linq/src/System/Linq/Index.cs b/src/libraries/System.Linq/src/System/Linq/Index.cs
index 49339b03d1ad39..11658cdafadcf4 100644
--- a/src/libraries/System.Linq/src/System/Linq/Index.cs
+++ b/src/libraries/System.Linq/src/System/Linq/Index.cs
@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
+using System.Diagnostics;
+using static System.Linq.Utilities;
 
 namespace System.Linq
 {
@@ -22,22 +24,11 @@ public static partial class Enumerable
             {
                 return [];
             }
-
-            return IndexIterator(source);
-        }
-
-        private static IEnumerable<(int Index, TSource Item)> IndexIterator<TSource>(IEnumerable<TSource> source)
-        {
-            int index = -1;
-            foreach (TSource element in source)
-            {
-                checked
-                {
-                    index++;
-                }
-
-                yield return (index, element);
-            }
+#if OPTIMIZE_FOR_SIZE
+            return new IEnumerableSelect2Iterator<TSource, (int Index, TSource Item)>(source, (x, i) => (i, x));
+#else
+            return new IEnumerableIndexIterator<TSource>(source);
+#endif
         }
     }
 }
diff --git a/src/libraries/System.Linq/src/System/Linq/Select.SpeedOpt.cs b/src/libraries/System.Linq/src/System/Linq/Select.SpeedOpt.cs
index f491f1f0de015a..afb239ebeb16bb 100644
--- a/src/libraries/System.Linq/src/System/Linq/Select.SpeedOpt.cs
+++ b/src/libraries/System.Linq/src/System/Linq/Select.SpeedOpt.cs
@@ -129,6 +129,156 @@ public override int GetCount(bool onlyIfCheap)
             }
         }
 
+        private sealed partial class IEnumerableSelect2Iterator<TSource, TResult>
+        {
+            public override TResult[] ToArray()
+            {
+                Func<TSource, int, TResult> selector = _selector;
+                int index = -1;
+
+                if (_source.TryGetNonEnumeratedCount(out int known))
+                {
+                    var array = new TResult[known];
+
+                    foreach (TSource item in _source)
+                    {
+                        array[checked(++index)] = selector(item, index);
+                    }
+
+                    return array;
+                }
+
+                SegmentedArrayBuilder<TResult>.ScratchBuffer scratch = default;
+                SegmentedArrayBuilder<TResult> builder = new(scratch);
+
+                foreach (TSource item in _source)
+                {
+                    builder.Add(selector(item, checked(++index)));
+                }
+
+                TResult[] result = builder.ToArray();
+                builder.Dispose();
+                return result;
+            }
+
+            public override List<TResult> ToList()
+            {
+                List<TResult> list = _source.TryGetNonEnumeratedCount(out int known) ? new(known) : [];
+                Func<TSource, int, TResult> selector = _selector;
+                int index = -1;
+
+                foreach (TSource item in _source)
+                {
+                    list.Add(selector(item, checked(++index)));
+                }
+
+                return list;
+            }
+
+            public override int GetCount(bool onlyIfCheap)
+            {
+                // In case someone uses Count() to force evaluation of
+                // the selector, run it provided `onlyIfCheap` is false.
+                if (onlyIfCheap)
+                {
+                    return _source.TryGetNonEnumeratedCount(out int known) ? known : -1;
+                }
+
+                int count = 0;
+
+                foreach (TSource item in _source)
+                {
+                    _selector(item, checked(count++));
+                }
+
+                return count;
+            }
+
+            public override TResult? TryGetElementAt(int index, out bool found)
+            {
+                if (_source is Iterator<TSource> iterator)
+                {
+                    return iterator.TryGetElementAt(index, out found) is var element && found
+                        ? _selector(element!, 0)
+                        : default;
+                }
+
+                if (index >= 0)
+                {
+                    IEnumerator<TSource> e = _source.GetEnumerator();
+                    int enumeratorIndex = -1;
+
+                    try
+                    {
+                        while (e.MoveNext())
+                        {
+                            if (index == 0)
+                            {
+                                found = true;
+                                return _selector(e.Current, checked(++enumeratorIndex));
+                            }
+
+                            index--;
+                        }
+                    }
+                    finally
+                    {
+                        (e as IDisposable)?.Dispose();
+                    }
+                }
+
+                found = false;
+                return default;
+            }
+
+            public override TResult? TryGetFirst(out bool found)
+            {
+                if (_source is Iterator<TSource> iterator)
+                {
+                    return iterator.TryGetFirst(out found) is var first && found ? _selector(first!, 0) : default;
+                }
+
+                using IEnumerator<TSource> e = _source.GetEnumerator();
+
+                if (e.MoveNext())
+                {
+                    found = true;
+                    return _selector(e.Current, 0);
+                }
+
+                found = false;
+                return default;
+            }
+
+            public override TResult? TryGetLast(out bool found)
+            {
+                if (_source is Iterator<TSource> iterator && iterator.GetCount(true) is not -1 and var count)
+                {
+                    return iterator.TryGetLast(out found) is var last && found ? _selector(last!, count - 1) : default;
+                }
+
+                using IEnumerator<TSource> e = _source.GetEnumerator();
+
+                if (e.MoveNext())
+                {
+                    found = true;
+                    TSource lastElement = e.Current;
+                    int lastIndex = -1;
+
+                    while (e.MoveNext())
+                    {
+                        lastElement = e.Current;
+                        lastIndex++;
+                    }
+
+                    return _selector(lastElement, lastIndex);
+                }
+
+                found = false;
+                return default;
+            }
+        }
+
         private sealed partial class ArraySelectIterator<TSource, TResult>
         {
             public override TResult[] ToArray()
diff --git a/src/libraries/System.Linq/src/System/Linq/Select.cs b/src/libraries/System.Linq/src/System/Linq/Select.cs
index 37a41c450f873c..683ee62ab0fcf2 100644
--- a/src/libraries/System.Linq/src/System/Linq/Select.cs
+++ b/src/libraries/System.Linq/src/System/Linq/Select.cs
@@ -68,21 +68,7 @@ public static IEnumerable<TResult> Select<TSource, TResult>(this IEnumerable<TSo
                 return [];
             }
 
-            return SelectIterator(source, selector);
-        }
-
-        private static IEnumerable<TResult> SelectIterator<TSource, TResult>(IEnumerable<TSource> source, Func<TSource, int, TResult> selector)
-        {
-            int index = -1;
-            foreach (TSource element in source)
-            {
-                checked
-                {
-                    index++;
-                }
-
-                yield return selector(element, index);
-            }
+            return new IEnumerableSelect2Iterator<TSource, TResult>(source, selector);
         }
 
         /// <summary>
@@ -145,6 +131,64 @@ public override IEnumerable<TResult2> Select<TResult2>(Func<TResult, TResult2> s
                 new IEnumerableSelectIterator<TSource, TResult2>(_source, CombineSelectors(_selector, selector));
         }
 
+        private sealed partial class IEnumerableSelect2Iterator<TSource, TResult> : Iterator<TResult>
+        {
+            private readonly IEnumerable<TSource> _source;
+            private readonly Func<TSource, int, TResult> _selector;
+            private int _index;
+            private IEnumerator<TSource>? _enumerator;
+
+            public IEnumerableSelect2Iterator(IEnumerable<TSource> source, Func<TSource, int, TResult> selector)
+            {
+                Debug.Assert(source is not null);
+                Debug.Assert(selector is not null);
+                _source = source;
+                _selector = selector;
+            }
+
+            private protected override Iterator<TResult> Clone() =>
+                new IEnumerableSelect2Iterator<TSource, TResult>(_source, _selector);
+
+            public override void Dispose()
+            {
+                if (_enumerator is not null)
+                {
+                    _enumerator.Dispose();
+                    _enumerator = null;
+                }
+
+                base.Dispose();
+            }
+
+            public override bool MoveNext()
+            {
+                switch (_state)
+                {
+                    case 1:
+                        _enumerator = _source.GetEnumerator();
+                        _index = -1;
+                        _state = 2;
+                        goto case 2;
+                    case 2:
+                        Debug.Assert(_enumerator is not null);
+
+                        if (_enumerator.MoveNext())
+                        {
+                            _current = _selector(_enumerator.Current, checked(++_index));
+                            return true;
+                        }
+
+                        Dispose();
+                        break;
+                }
+
+                return false;
+            }
+
+            public override IEnumerable<TResult2> Select<TResult2>(Func<TResult, TResult2> selector) =>
+                new IEnumerableSelect2Iterator<TSource, TResult2>(_source, CombineSelectors(_selector, selector));
+        }
+
         /// <summary>
         /// An iterator that maps each item of an array.
         /// </summary>
diff --git a/src/libraries/System.Linq/src/System/Linq/Utilities.cs b/src/libraries/System.Linq/src/System/Linq/Utilities.cs
index 987d0004d4ec3a..9cc8245e17f1a2 100644
--- a/src/libraries/System.Linq/src/System/Linq/Utilities.cs
+++ b/src/libraries/System.Linq/src/System/Linq/Utilities.cs
@@ -69,5 +69,19 @@ public static Func<TSource, bool> CombinePredicates<TSource>(Func<TSource, bool>
         /// </returns>
         public static Func<TSource, TResult> CombineSelectors<TSource, TMiddle, TResult>(Func<TSource, TMiddle> selector1, Func<TMiddle, TResult> selector2) =>
             x => selector2(selector1(x));
+
+        /// <summary>
+        /// Combines two selectors.
+        /// </summary>
+        /// <typeparam name="TSource">The type of the first selector's argument.</typeparam>
+        /// <typeparam name="TMiddle">The type of the second selector's argument.</typeparam>
+        /// <typeparam name="TResult">The type of the second selector's return value.</typeparam>
+        /// <param name="selector1">The first selector to run.</param>
+        /// <param name="selector2">The second selector to run.</param>
+        /// <returns>
+        /// A new selector that represents the composition of the first selector with the second selector.
+        /// </returns>
+        public static Func<TSource, int, TResult> CombineSelectors<TSource, TMiddle, TResult>(Func<TSource, int, TMiddle> selector1, Func<TMiddle, TResult> selector2) =>
+            (x, i) => selector2(selector1(x, i));
     }
 }