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
int compare = internalCompare(comparator, parentPath, data, element);
			if (compare == 0) {
				
				while (compare == 0) {
					++mid;
					if (mid >= count) {
						break;
					}
					data = items[mid].getData();
					compare = internalCompare(comparator, parentPath, data,
							element);
				}
				return mid;
			}
			if (compare < 0) {
				min = mid + 1;
			} else {
				max = mid - 1;
			}
		}
		return min;
	}
	/**
	 * Return the tree path that should be used as the parent path for the given
	 * widget and sorter. A <code>null</code> is returned if either the sorter
	 * is not a {@link TreePathViewerSorter} or if the parent widget is not an
	 * {@link Item} (i.e. is the root of the tree).
	 * 
	 * @param parent
	 *            the parent widget