Hi there.
I have a page with a jstree and/or jqgrid - it really doesn't matter which one I use to show my point, as I suspect it's any hover effect in IE8... Back to point:
When hovering over an <a> tag the background-color is changed with CSS. In all browsers, including IE7 (although slightly slower - I'd guess +-300ms) there is no lag in the background change, but in IE8 the lag can get up to, and over, 1 second.
- I do not call any extra - onmouseover,onmouseout, etc.. on any of these elements.
- I've turned off all animations in jQuery. 
- Hiding all elements on the page except for the jstree(for example) does make the hover effect slightly faster, but nothing really noticable. 
- Using the IE8 Debuggers profiler does not show any significant overhead, only normal times in the jQuery sizzle.js are noticeable 
- This is after all document elements have been loaded and no extra calls are made to servers, no new images are downloaded,etc.. 
Any advice on improving this, eg: performance tests, tag selection, etc.. would be appreciated greatly.
Regards, Byron Cobb.
P.S. This question asked previously by bobmoff didn't really answer anything.
EDIT:
Further tests have shown that the jquery .hover() and binding mouseenter and mouseleave are also equally as slow in IE8. For example - I have removed everything on the page except for my jqgrid and the hover effect is still slow(although a slight increase is observed)
Running IE8 in compatibility mode also fixes this slow hover issue.