diff --git a/src/libraries/Common/src/System/Collections/Generic/LargeArrayBuilder.SizeOpt.cs b/src/libraries/Common/src/System/Collections/Generic/LargeArrayBuilder.SizeOpt.cs
index 4ec34a701d5232..181d2fd282afde 100644
--- a/src/libraries/Common/src/System/Collections/Generic/LargeArrayBuilder.SizeOpt.cs
+++ b/src/libraries/Common/src/System/Collections/Generic/LargeArrayBuilder.SizeOpt.cs
@@ -35,8 +35,6 @@ public void AddRange(IEnumerable<T> items)
             }
         }
 
-        public void SlowAdd(T item) => _builder.Add(item);
-
         public T[] ToArray() => _builder.ToArray();
 
         public CopyPosition CopyTo(CopyPosition position, T[] array, int arrayIndex, int count)
diff --git a/src/libraries/Common/tests/TestUtilities/System/PlatformDetection.cs b/src/libraries/Common/tests/TestUtilities/System/PlatformDetection.cs
index dfd4ba7b817626..f56404ff270243 100644
--- a/src/libraries/Common/tests/TestUtilities/System/PlatformDetection.cs
+++ b/src/libraries/Common/tests/TestUtilities/System/PlatformDetection.cs
@@ -50,6 +50,9 @@ public static partial class PlatformDetection
         public static bool IsThreadingSupported => !IsBrowser;
         public static bool IsBinaryFormatterSupported => !IsBrowser;
 
+        public static bool IsSpeedOptimized => !IsSizeOptimized;
+        public static bool IsSizeOptimized => IsBrowser || IsAndroid || IsiOS || IstvOS;
+
         public static bool IsBrowserDomSupported => GetIsBrowserDomSupported();
         public static bool IsNotBrowserDomSupported => !IsBrowserDomSupported;
 
diff --git a/src/libraries/System.Linq/src/System.Linq.csproj b/src/libraries/System.Linq/src/System.Linq.csproj
index ea585685f3e609..2f814694d91f56 100644
--- a/src/libraries/System.Linq/src/System.Linq.csproj
+++ b/src/libraries/System.Linq/src/System.Linq.csproj
@@ -1,9 +1,17 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>$(NetCoreAppCurrent)</TargetFrameworks>
+    <TargetFrameworks>$(NetCoreAppCurrent);$(NetCoreAppCurrent)-Browser;$(NetCoreAppCurrent)-Android;$(NetCoreAppCurrent)-iOS;$(NetCoreAppCurrent)-tvOS</TargetFrameworks>
     <Nullable>enable</Nullable>
+    <OptimizeForSize Condition="'$(TargetsBrowser)' == 'true' or '$(TargetsAndroid)' == 'true' or '$(TargetsiOS)' == 'true' or '$(TargetstvOS)' == 'true'">true</OptimizeForSize>
   </PropertyGroup>
-  <ItemGroup> <!-- Optimize for speed -->
+  <ItemGroup Condition="'$(OptimizeForSize)' == true">
+    <Compile Include="System\Linq\Enumerable.SizeOpt.cs" />
+    <Compile Include="System\Linq\Skip.SizeOpt.cs" />
+    <Compile Include="System\Linq\Take.SizeOpt.cs" />
+    <Compile Include="$(CommonPath)\System\Collections\Generic\LargeArrayBuilder.SizeOpt.cs"
+             Link="System\Collections\Generic\LargeArrayBuilder.SizeOpt.cs" />
+  </ItemGroup>
+  <ItemGroup Condition="'$(OptimizeForSize)' != true">
     <Compile Include="System\Linq\AppendPrepend.SpeedOpt.cs" />
     <Compile Include="System\Linq\Concat.SpeedOpt.cs" />
     <Compile Include="System\Linq\DefaultIfEmpty.SpeedOpt.cs" />
diff --git a/src/libraries/System.Linq/tests/ConcatTests.cs b/src/libraries/System.Linq/tests/ConcatTests.cs
index 256fea4aaa14e5..6209d846ff2438 100644
--- a/src/libraries/System.Linq/tests/ConcatTests.cs
+++ b/src/libraries/System.Linq/tests/ConcatTests.cs
@@ -244,7 +244,7 @@ public static IEnumerable<object[]> ManyConcatsData()
             yield return new object[] { Enumerable.Range(0, 500).Select(i => Enumerable.Repeat(i, 1)).Reverse() };
         }
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsSpeedOptimized))]
         public void CountOfConcatIteratorShouldThrowExceptionOnIntegerOverflow()
         {
             var supposedlyLargeCollection = new DelegateBasedCollection<int> { CountWorker = () => int.MaxValue };
diff --git a/src/libraries/System.Linq/tests/EmptyPartitionTests.cs b/src/libraries/System.Linq/tests/EmptyPartitionTests.cs
index 7417cbe957dec8..c98884ee067000 100644
--- a/src/libraries/System.Linq/tests/EmptyPartitionTests.cs
+++ b/src/libraries/System.Linq/tests/EmptyPartitionTests.cs
@@ -29,7 +29,7 @@ public void SingleInstance()
             Assert.True(ReferenceEquals(GetEmptyPartition<int>(), GetEmptyPartition<int>()));
         }
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsSpeedOptimized))]
         [SkipOnTargetFramework(~TargetFrameworkMonikers.Netcoreapp, ".NET Core returns the instance as an optimization")]
         public void SkipSame()
         {
@@ -37,7 +37,7 @@ public void SkipSame()
             Assert.Same(empty, empty.Skip(2));
         }
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsSpeedOptimized))]
         [SkipOnTargetFramework(~TargetFrameworkMonikers.Netcoreapp, ".NET Core returns the instance as an optimization")]
         public void TakeSame()
         {
diff --git a/src/libraries/System.Linq/tests/OrderedSubsetting.cs b/src/libraries/System.Linq/tests/OrderedSubsetting.cs
index 1e282147bb50a0..f4ec16f97cc87b 100644
--- a/src/libraries/System.Linq/tests/OrderedSubsetting.cs
+++ b/src/libraries/System.Linq/tests/OrderedSubsetting.cs
@@ -224,7 +224,7 @@ public void TakeAndSkip()
             Assert.Equal(Enumerable.Range(10, 1), ordered.Take(11).Skip(10));
         }
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsSpeedOptimized))]
         [SkipOnTargetFramework(~TargetFrameworkMonikers.Netcoreapp, "This fails with an OOM, as it iterates through the large array. See https://github.com/dotnet/corefx/pull/6821.")]
         public void TakeAndSkip_DoesntIterateRangeUnlessNecessary()
         {
diff --git a/src/libraries/System.Linq/tests/RangeTests.cs b/src/libraries/System.Linq/tests/RangeTests.cs
index 83527a918af3d9..68894cf8052405 100644
--- a/src/libraries/System.Linq/tests/RangeTests.cs
+++ b/src/libraries/System.Linq/tests/RangeTests.cs
@@ -211,14 +211,14 @@ public void FirstOrDefault()
             Assert.Equal(-100, Enumerable.Range(-100, int.MaxValue).FirstOrDefault());
         }
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsSpeedOptimized))]
         [SkipOnTargetFramework(~TargetFrameworkMonikers.Netcoreapp, ".NET Core optimizes Enumerable.Range().Last(). Without this optimization, this test takes a long time. See https://github.com/dotnet/corefx/pull/2401.")]
         public void Last()
         {
             Assert.Equal(1000000056, Enumerable.Range(57, 1000000000).Last());
         }
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsSpeedOptimized))]
         [SkipOnTargetFramework(~TargetFrameworkMonikers.Netcoreapp, ".NET Core optimizes Enumerable.Range().LastOrDefault(). Without this optimization, this test takes a long time. See https://github.com/dotnet/corefx/pull/2401.")]
         public void LastOrDefault()
         {
diff --git a/src/libraries/System.Linq/tests/SelectManyTests.cs b/src/libraries/System.Linq/tests/SelectManyTests.cs
index 45c5a7da236608..50d97f07bb291f 100644
--- a/src/libraries/System.Linq/tests/SelectManyTests.cs
+++ b/src/libraries/System.Linq/tests/SelectManyTests.cs
@@ -464,7 +464,7 @@ public static IEnumerable<object[]> DisposeAfterEnumerationData()
             return lengths.SelectMany(l => lengths, (l1, l2) => new object[] { l1, l2 });
         }
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsSpeedOptimized))]
         [SkipOnTargetFramework(~TargetFrameworkMonikers.Netcoreapp, ".NET Core optimizes SelectMany and throws an OverflowException. On the .NET Framework this takes a long time. See https://github.com/dotnet/corefx/pull/13942.")]
         [InlineData(new[] { int.MaxValue, 1 })]
         [InlineData(new[] { 2, int.MaxValue - 1 })]
diff --git a/src/libraries/System.Linq/tests/TakeTests.cs b/src/libraries/System.Linq/tests/TakeTests.cs
index a9580fb22d3c34..239ec1f9ed716c 100644
--- a/src/libraries/System.Linq/tests/TakeTests.cs
+++ b/src/libraries/System.Linq/tests/TakeTests.cs
@@ -459,7 +459,7 @@ public void RepeatEnumeratingNotList()
             Assert.Equal(taken, taken);
         }
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsSpeedOptimized))]
         [InlineData(1000)]
         [InlineData(1000000)]
         [InlineData(int.MaxValue)]