I'm experiencing the same, multi-second gui response times on a recent laptop, and I think it's not related to Debian Lenny only, but to problems with the intel x.org driver in Ubuntu intrepid, jaunty and quite possibly Debian Lenny.

I don't have any hard evidence other than that I've seen the exact same problem happening for other applications that do something fancy graphics-wise, or java applications like jedit. I asked lcuk on IRC yesterday, and he confirmed he is also running Intrepid like me.