I have an unbound grid with 30+ columns and 96 rows.  I first thought there was an issue with scrolling as I found I had to wait until I could scroll the grid each time I moved from the horizontal to the vertical scroll bar and vice versa. I then found that this was actually in issue with moving over grid cells.

The scrollbar issue only happened if I moved the cursor across the grid to the slider.  If I moved it round the edge of the grid then the sliders worked fine.  So it is not moving over scrollbars but moving over cells that causes the high CPU. 

I have also found that if I move over cells I have moved over before, then a lot less CPU is used.  I only have to move over one 'new' cell for the 'CPU wait' to happen.  I can't see that this can be related to my MouseOverHandler?  Interestingly if I leave the page for a minute or two and then come back to it everything works smoothly.  That is until I scroll to a new area of the grid when the same thing happens again.

I have no issues with IE7. This looks like some IE8 background/caching process. Subjectively the initial grid rendering looks faster in IE8.

To be fair I have only seen this issue on an older P4 2.66Ghz laptop. The CPU peak occurs on faster machines is quite big but not so noticeable.  There may not be much that can be done but some of our customers have older machines and will take all updates that are thrown at them, including IE8, without thought. 

If this is an issue or if there is something I can do in IE8 or code, I would like to know.  Just understanding what is happening would be useful.