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
if (!getExpanded(parentItem) && getItemCount(parentItem) == 0) {
				
				if (isExpandable(parentItem, null, parentItem.getData())) {
					newItem(parentItem, SWT.NULL, -1);
				} else {
					
					tree.redraw();
				}
			}
		}
	}
	/**
	 * Removes the given elements from this viewer, whenever those elements
	 * appear as children of the given parent.
	 * 
	 * @param elements
	 *            the elements to remove
	 * @since 3.1
	 */
	protected void internalRemove(Object parent, Object[] elements) {
		CustomHashtable toRemove = new CustomHashtable(getComparer());
		for (int i = 0; i < elements.length; i++) {
			toRemove.put(elements[i], elements[i]);
		}
		
		
		CustomHashtable parentItems = new CustomHashtable(5);
		
		Widget[] parentItemArray = findItems(parent);
		for (int i = 0; i < parentItemArray.length; i++) {