Bug 161086 [Viewers] performance problem in AbstractTreeViewer.internalExpandToLevel
If you expand to ALL_LEVELS, internalExpandToLevel will call createChildren even if the element has no children, i.e. it does not check if the element is expandable. Found this while experimenting with the Quick Access feature (enter "con", then delete all three characters one at a time).
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/AbstractTreeViewer.java
Widget parentItem = parentItemArray[i];
			if (parentItem instanceof Item) {
				parentItems.put(parentItem, parentItem);
			}
			
			Item[] children = getChildren(parentItem);
			for (int j = 0; j < children.length; j++) {
				Item child = children[j];
				Object data = child.getData();
				if (data != null && toRemove.containsKey(data)) {
					disassociate(child);
					child.dispose();
				}
			}
		}
		internalReconcileParentItems(parentItems);
	}
	/**
	 * Sets the expanded state of all items to correspond to the given set of
	 * expanded elements.
	 * 
	 * @param expandedElements
	 *            the set (element type: <code>Object</code>) of elements
	 *            which are expanded
	 * @param widget
	 *            the widget
	 */
	private void internalSetExpanded(CustomHashtable expandedElements,
			Widget widget) {
		Item[] items = getChildren(widget);