There's way too much processing going on when moving the mouse over a tree or grid. If I record half a second of moving my mouse over a row or two, I see _thousands_ of function calls logged in the FB profiler. Try this in Chrome, you'll see a noticeable lag with the row hover effect - which is insane, considering it's Chrome, and a Tree with 10 rows. Only option seems to be to disable trackOver to avoid the poor impression.
It's _perceived_ as extremely sluggish. Do a side by side with 3.x and you'll feel it, huge difference. Couldn't the hover effect just use good old :hover for good browsers to save all the event firing?