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
* </p>
	 * 
	 * @param elementOrTreePath
	 *            the element or path
	 * @return <code>true</code> if the tree node representing the given
	 *         element can be expanded, or <code>false</code> if not
	 */
	public boolean isExpandable(Object elementOrTreePath) {
		Object element;
		TreePath path;
		if (elementOrTreePath instanceof TreePath) {
			path = (TreePath) elementOrTreePath;
			element = path.getLastSegment();
		} else {
			element = elementOrTreePath;
			path = null;
		}
		IContentProvider cp = getContentProvider();
		if (cp instanceof ITreePathContentProvider) {
			ITreePathContentProvider tpcp = (ITreePathContentProvider) cp;
			if (path == null) {
				
				Widget w = findItem(element);
				if (w instanceof Item) {
					Item item = (Item) w;
					path = getTreePathFromItem(item);
				}
				if (path == null) {
					path = new TreePath(new Object[] { element });
				}