diff --git a/tlatools/org.lamport.tlatools/src/tlc2/debug/TLCStackFrame.java b/tlatools/org.lamport.tlatools/src/tlc2/debug/TLCStackFrame.java
index 936e4882f4..ff440024f5 100644
--- a/tlatools/org.lamport.tlatools/src/tlc2/debug/TLCStackFrame.java
+++ b/tlatools/org.lamport.tlatools/src/tlc2/debug/TLCStackFrame.java
@@ -25,7 +25,6 @@
  ******************************************************************************/
 package tlc2.debug;
 
-import java.io.File;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
@@ -49,6 +48,9 @@
 
 class TLCStackFrame extends StackFrame {
 	
+	// Not thread-safe because TLCDebugger is assumed to take care of synchronization!
+	private static final Map<SemanticNode, String> PATH_CACHE = new HashMap<>();
+	
 	public static final String SCOPE = "Context";
 	
 	// It would be easier to use hashCode instead of passing a random generator
@@ -90,8 +92,10 @@ public TLCStackFrame(SemanticNode node, Context ctxt, final Tool tool) {
 
 		final Source source = new Source();
 		source.setName(node.getLocation().source());
-		final File moduleFile = tool.getResolver().resolve(node.getTreeNode().getFilename(), true);
-		source.setPath(moduleFile.getAbsolutePath().toString());
+		// resolve(..) triggers a file-system round-trip (IO), which is obviously too
+		// expensive!!! Thus, cache the result.
+		source.setPath(PATH_CACHE.computeIfAbsent(node,
+				n -> tool.getResolver().resolve(n.getTreeNode().getFilename(), true).getAbsolutePath().toString()));
 		setSource(source);
 		
 		this.stackId = rnd.nextInt(Integer.MAX_VALUE - 1) + 1;