diff --git a/src/Main/ICSharpCode.SharpDevelop.Dom/Project/Src/CtrlSpaceResolveHelper.cs b/src/Main/ICSharpCode.SharpDevelop.Dom/Project/Src/CtrlSpaceResolveHelper.cs
index 473b422cf2..11671b10fa 100644
--- a/src/Main/ICSharpCode.SharpDevelop.Dom/Project/Src/CtrlSpaceResolveHelper.cs
+++ b/src/Main/ICSharpCode.SharpDevelop.Dom/Project/Src/CtrlSpaceResolveHelper.cs
@@ -108,17 +108,19 @@ namespace ICSharpCode.SharpDevelop.Dom
 			if (callingClass == null) {
 				return;
 			}
+			// use HashSet so that Contains lookups are possible in O(1).
+			HashSet<ICompletionEntry> existingResults = new HashSet<ICompletionEntry>(result);
 			string[] namespaceParts = callingClass.Namespace.Split('.');
 			for (int i = 1; i <= namespaceParts.Length; i++) {
 				foreach (ICompletionEntry member in projectContent.GetNamespaceContents(string.Join(".", namespaceParts, 0, i))) {
-					if (!result.Contains(member))
+					if (!existingResults.Contains(member))
 						result.Add(member);
 				}
 			}
 			IClass currentClass = callingClass;
 			do {
 				foreach (IClass innerClass in currentClass.GetCompoundClass().GetAccessibleTypes(currentClass)) {
-					if (!result.Contains(innerClass))
+					if (!existingResults.Contains(innerClass))
 						result.Add(innerClass);
 				}
 				currentClass = currentClass.DeclaringType;