It certainly is a javascript problem, we had a fix that prevented javascript from piling up under some circumstances. You can see an example of this massive slowdown by going to the calendar and clicking the next month arrow, after about 5 clicks browsers will slow to a crawl.
In 13.1 we fixed this issue but it seems IE 8&9 were negatively impacted. We are working on this at the moment.
Matt Petty
Senior Software Developer
SmarterTools Inc.
www.smartertools.com