diff --git a/src/Microsoft.TestPlatform.CrossPlatEngine/Discovery/DiscoveryResultCache.cs b/src/Microsoft.TestPlatform.CrossPlatEngine/Discovery/DiscoveryResultCache.cs
index 63de79957a..9b9e8002ce 100644
--- a/src/Microsoft.TestPlatform.CrossPlatEngine/Discovery/DiscoveryResultCache.cs
+++ b/src/Microsoft.TestPlatform.CrossPlatEngine/Discovery/DiscoveryResultCache.cs
@@ -60,7 +60,7 @@ public DiscoveryResultCache(long cacheSize, TimeSpan discoveredTestEventTimeout,
         _lastUpdate = DateTime.UtcNow;
         _cacheTimeout = discoveredTestEventTimeout;
 
-        _tests = new List<TestCase>();
+        _tests = new List<TestCase>(InitialCapacity(cacheSize));
         TotalDiscoveredTests = 0;
     }
 
@@ -112,11 +112,13 @@ public void AddTest(TestCase test)
             {
                 // Pass on the buffer to the listener and clear the old one
                 _onReportTestCases(_tests);
-                _tests = new List<TestCase>();
+                _tests = new List<TestCase>(InitialCapacity(_cacheSize));
                 _lastUpdate = DateTime.UtcNow;
 
                 EqtTrace.Verbose("DiscoveryResultCache.AddTest: Notified the onReportTestCases callback.");
             }
         }
     }
+
+    private static int InitialCapacity(long cacheSize) => (int)Math.Min(cacheSize, 512);
 }
diff --git a/src/Microsoft.TestPlatform.CrossPlatEngine/Execution/TestRunCache.cs b/src/Microsoft.TestPlatform.CrossPlatEngine/Execution/TestRunCache.cs
index bf680a873a..ba177ef0f7 100644
--- a/src/Microsoft.TestPlatform.CrossPlatEngine/Execution/TestRunCache.cs
+++ b/src/Microsoft.TestPlatform.CrossPlatEngine/Execution/TestRunCache.cs
@@ -3,7 +3,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Collections.ObjectModel;
 using System.Linq;
 using System.Threading;
 
@@ -60,12 +59,12 @@ internal class TestRunCache : ITestRunCache
     /// <summary>
     /// The test case currently in progress.
     /// </summary>
-    private ICollection<TestCase> _inProgressTests;
+    private List<TestCase> _inProgressTests;
 
     /// <summary>
     /// Test results buffer
     /// </summary>
-    private ICollection<TestResult> _testResults;
+    private List<TestResult> _testResults;
 
     /// <summary>
     /// Sync object
@@ -93,8 +92,8 @@ internal TestRunCache(long cacheSize, TimeSpan cacheTimeout, OnCacheHit onCacheH
         _onCacheHit = onCacheHit;
         _lastUpdate = DateTime.UtcNow;
         _cacheTimeout = cacheTimeout;
-        _inProgressTests = new Collection<TestCase>();
-        _testResults = new Collection<TestResult>();
+        _inProgressTests = new List<TestCase>(InitialCapacity(cacheSize));
+        _testResults = new List<TestResult>(InitialCapacity(cacheSize));
         _runStats = new Dictionary<TestOutcome, long>();
         _syncObject = new object();
 
@@ -272,7 +271,10 @@ public ICollection<TestResult> GetLastChunk()
         {
             var lastChunk = _testResults;
 
-            _testResults = new Collection<TestResult>();
+            // GetLastChunk() is the end-of-run drain; no further results are expected after this
+            // call, so avoid pre-allocating capacity here. The replacement list exists only to keep
+            // the field non-null and will be allocated lazily if results somehow arrive afterwards.
+            _testResults = new List<TestResult>();
 
             return lastChunk;
         }
@@ -337,8 +339,8 @@ private void SendResults()
     {
         // Pass on the buffer to the listener and clear the old one
         _onCacheHit(TestRunStatistics, _testResults, _inProgressTests);
-        _testResults = new Collection<TestResult>();
-        _inProgressTests = new Collection<TestCase>();
+        _testResults = new List<TestResult>(InitialCapacity(_cacheSize));
+        _inProgressTests = new List<TestCase>(InitialCapacity(_cacheSize));
         _lastUpdate = DateTime.UtcNow;
 
         // Reset the timer
@@ -371,4 +373,6 @@ private void RemoveInProgress(TestResult result)
         }
     }
 
+    private static int InitialCapacity(long cacheSize) => (int)Math.Min(cacheSize, 512);
+
 }