Probably a bit, but I never noticed that much difference. 3 year old computers should be fine anyway. Past that it's hard to say, but I think if it can run XP, it can probably run IE8. If it can't run XP you wouldn't be able to install IE8 anyway.
I have noticed IE8 is noticeably slower than IE7, especially when opening a new instance or a new tab; even on the new Lenovo PCs we are getting at work.