getBoundingClientRect() is slow in ie

the problem is reproduced in zk fiddle:
http://zkfiddle.org/sample/2h2q4u2/6-Performance-problem-with-TreeDataEvent-INTERVAL-REMOVED

click "Remove a bunch of tree items" button in ie. It will take a long time to remove tree items.

Workaround:
setting mold="paging" with a large page size will avoid the problem (by automatically calling invalidate() on the tree)

We do some profiling and find most of time are spent on getBoundingClientRect(). In IE getBoundingClientRect() takes ~10x as long as chrome.