I've actually been noticing a similar issue on my own computer (IE6 on XP SP3). Everything goes smoothly at first, but after awhile, I start experiencing lag when I highlight anything. It's worse when trying to use the context menu, as Karnac has described. In fact, I just messed around with it and this is what happened...
I highlighted a couple of words and right-clicked.
About 5-8 seconds later, the context menu finally showed up.
I clicked on Copy and it took a few more seconds for the context menu to go away.
I then had to wait an additional 3-5 seconds for my cursor to become response. When hovering over a hyperlink, I had to wait for it to switch from an arrow to a hand.
From then on, the lag was significantly worse than before, even if only highlighting without any action.
The amount of text highlighted doesn't appear to have any impact on this. However, it does seem to differ from page to page. For instance, when replying to a post, there isn't any lag at all. In fact, it even seems to somehow "reset" the problem temporarily. It could just be a fluke for Karnac and I, but it appears to me that this might not be IE-friendly. Maybe it would be a good idea to have a few people test it in IE to see if anyone else has any issues.
