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
CustomHashtable expandedTreePaths, Widget widget,
			TreePath currentPath) {
		Item[] items = getChildren(widget);
		for (int i = 0; i < items.length; i++) {
			Item item = items[i];
			Object data = item.getData();
			TreePath childPath = data == null ? null : currentPath
					.createChildPath(data);
			if (data != null && childPath != null) {
				
				
				boolean expanded = expandedTreePaths.remove(childPath) != null;
				if (expanded != getExpanded(item)) {
					if (expanded) {
						createChildren(item);
					}
					setExpanded(item, expanded);
				}
			}
			internalSetExpandedTreePaths(expandedTreePaths, item, childPath);
		}
	}
	/**
	 * Return whether the tree node representing the given element or path can
	 * be expanded. Clients should query expandablity by path if the viewer's
	 * content provider is an {@link ITreePathContentProvider}.
	 * <p>
	 * The default implementation of this framework method calls
	 * <code>hasChildren</code> on this viewer's content provider. It may be
	 * overridden if necessary.