Index: src/org/openstreetmap/josm/gui/dialogs/CommandStackDialog.java
===================================================================
--- src/org/openstreetmap/josm/gui/dialogs/CommandStackDialog.java	(revision 14829)
+++ src/org/openstreetmap/josm/gui/dialogs/CommandStackDialog.java	(working copy)
@@ -89,6 +89,8 @@
     private final SelectAction selectAction = new SelectAction();
     private final SelectAndZoomAction selectAndZoomAction = new SelectAndZoomAction();
 
+    private boolean passive;
+
     /**
      * Constructs a new {@code CommandStackDialog}.
      */
@@ -550,4 +552,23 @@
             add(selectAndZoomAction);
         }
     }
+
+    /**
+     * Allows to set the dialog into passive mode so that updates to the undo/redo tree are ignored
+     * @param b true: enable passive mode, false: disable passive mode and rebuild dialog
+     */
+    public void setPassive(boolean b) {
+        if (passive) {
+            if (!b) {
+                passive = false;
+                UndoRedoHandler.getInstance().addCommandQueuePreciseListener(this);
+                buildTrees();
+            }
+        } else {
+            passive = b;
+            if (passive) {
+                UndoRedoHandler.getInstance().removeCommandQueuePreciseListener(this);
+            }
+        }
+    }
 }
Index: src/org/openstreetmap/josm/gui/dialogs/ValidatorDialog.java
===================================================================
--- src/org/openstreetmap/josm/gui/dialogs/ValidatorDialog.java	(revision 14829)
+++ src/org/openstreetmap/josm/gui/dialogs/ValidatorDialog.java	(working copy)
@@ -618,7 +618,11 @@
         @Override
         protected void realRun() throws SAXException, IOException, OsmTransferException {
             ProgressMonitor monitor = getProgressMonitor();
+            CommandStackDialog cmdStackPanel = dialogsPanel.getToggleDialog(CommandStackDialog.class);
             try {
+                if (cmdStackPanel != null && testErrors.size() > 100) {
+                    cmdStackPanel.setPassive(true);
+                }
                 monitor.setTicksCount(testErrors.size());
                 final DataSet ds = MainApplication.getLayerManager().getActiveDataSet();
                 int i = 0;
@@ -646,6 +650,9 @@
                 throw new JosmRuntimeException(e);
             } finally {
                 monitor.finishTask();
+                if (cmdStackPanel != null) {
+                    cmdStackPanel.setPassive(false);
+                }
             }
         }
     }