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
Item current = getItem(x, y);
		if (current != null) {
			Item previous = getPreviousItem(current);
			showItem(previous == null ? current : previous);
			return previous;
		}
		return null;
	}
	/**
	 * Sets the auto-expand level. The value 0 means that there is no
	 * auto-expand; 1 means that top-level elements are expanded, but not their
	 * children; 2 means that top-level elements are expanded, and their
	 * children, but not grandchildren; and so on.
	 * <p>
	 * The value <code>ALL_LEVELS</code> means that all subtrees should be
	 * expanded.
	 * </p>
	 * 
	 * @param level
	 *            non-negative level, or <code>ALL_LEVELS</code> to expand all
	 *            levels of the tree
	 */
	public void setAutoExpandLevel(int level) {
		expandToLevel = level;
	}
	/**
	 * The <code>AbstractTreeViewer</code> implementation of this method
	 * checks to ensure that the content provider is an
	 * <code>ITreeContentProvider</code>.
	 */