diff --git a/ILSpy/AssemblyList.cs b/ILSpy/AssemblyList.cs
index 9585d71f0..d49ed3c23 100644
--- a/ILSpy/AssemblyList.cs
+++ b/ILSpy/AssemblyList.cs
@@ -41,6 +41,8 @@ namespace ICSharpCode.ILSpy
 		/// <summary>Dirty flag, used to mark modifications so that the list is saved later</summary>
 		bool dirty;
 		
+		internal readonly ConcurrentDictionary<string, LoadedAssembly> assemblyLookupCache = new ConcurrentDictionary<string, LoadedAssembly>();
+		
 		/// <summary>
 		/// The assemblies in this list.
 		/// Needs locking for multi-threaded access!
@@ -101,6 +103,7 @@ namespace ICSharpCode.ILSpy
 		
 		void Assemblies_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
 		{
+			assemblyLookupCache.Clear();
 			// Whenever the assembly list is modified, mark it as dirty
 			// and enqueue a task that saves it once the UI has finished modifying the assembly list.
 			if (!dirty) {
@@ -111,6 +114,7 @@ namespace ICSharpCode.ILSpy
 						delegate {
 							dirty = false;
 							AssemblyListManager.SaveList(this);
+							assemblyLookupCache.Clear();
 						})
 				);
 			}
diff --git a/ILSpy/LoadedAssembly.cs b/ILSpy/LoadedAssembly.cs
index 147044157..fdfb2636b 100644
--- a/ILSpy/LoadedAssembly.cs
+++ b/ILSpy/LoadedAssembly.cs
@@ -17,11 +17,11 @@
 // DEALINGS IN THE SOFTWARE.
 
 using System;
+using System.Collections.Concurrent;
 using System.IO;
 using System.Linq;
 using System.Threading.Tasks;
 using System.Windows.Threading;
-
 using Mono.Cecil;
 
 namespace ICSharpCode.ILSpy
@@ -133,6 +133,8 @@ namespace ICSharpCode.ILSpy
 				if (!disposed) {
 					disposed = true;
 					assemblyLoadDisableCount--;
+					// clear the lookup cache since we might have stored the lookups failed due to DisableAssemblyLoad()
+					MainWindow.Instance.CurrentAssemblyList.assemblyLookupCache.Clear();
 				}
 			}
 		}
@@ -177,6 +179,11 @@ namespace ICSharpCode.ILSpy
 		}
 		
 		public LoadedAssembly LookupReferencedAssembly(string fullName)
+		{
+			return assemblyList.assemblyLookupCache.GetOrAdd(fullName, LookupReferencedAssemblyInternal);
+		}
+		
+		LoadedAssembly LookupReferencedAssemblyInternal(string fullName)
 		{
 			foreach (LoadedAssembly asm in assemblyList.GetAssemblies()) {
 				if (asm.AssemblyDefinition != null && fullName.Equals(asm.AssemblyDefinition.FullName, StringComparison.OrdinalIgnoreCase))