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
public void setExpandedElements(Object[] elements) {
		assertElementsNotNull(elements);
		CustomHashtable expandedElements = newHashtable(elements.length * 2 + 1);
		for (int i = 0; i < elements.length; ++i) {
			Object element = elements[i];
			
			
			
			
			internalExpand(element, false);
			expandedElements.put(element, element);
		}
		
		
		
		
		internalSetExpanded(expandedElements, getControl());
	}
	/**
	 * Sets which nodes are expanded in this viewer's tree. The given list
	 * contains the tree paths that are to be expanded; all other nodes are to
	 * be collapsed.
	 * <p>
	 * This method is typically used when restoring the interesting state of a
	 * viewer captured by an earlier call to <code>getExpandedTreePaths</code>.
	 * </p>
	 * 
	 * @param treePaths
	 *            the array of expanded tree paths
	 * @see #getExpandedTreePaths()