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
*/
	protected void internalExpandToLevel(Widget widget, int level) {
		if (level == ALL_LEVELS || level > 0) {
			createChildren(widget);
			if (widget instanceof Item) {
				setExpanded((Item) widget, true);
			}
			if (level == ALL_LEVELS || level > 1) {
				Item[] children = getChildren(widget);
				if (children != null) {
					int newLevel = (level == ALL_LEVELS ? ALL_LEVELS
							: level - 1);
					for (int i = 0; i < children.length; i++) {
						internalExpandToLevel(children[i], newLevel);
					}
				}
			}
		}
	}
	/**
	 * Non-recursively tries to find the given element as a child of the given
	 * parent (item or tree).
	 * 
	 * @param parent
	 *            the parent item
	 * @param element
	 *            the element
	 * @return Widget
	 */
	private Widget internalFindChild(Widget parent, Object element) {