I saw the same keyboard lagging with kubuntu intrepid (8.10/KDE 4.1). It was horrible. And it happened with all effects turned off on a state-of-the-art laptop at that time (the Dell Latitude which was just released in August 2008). It was so bad that you couldn't even use the terminal (konsole). Sometimes it was not there directly after rebooting, but after a few minutes using it, it suddenly kicked in.
In my case, this was really, really, horrible. I was really used to kubuntu, and I wasn't a very experienced linux user, yet. However, there was NO way for me to continue using kubuntu on this machine. It needed a recent kernel to get the new hardware supported; so using kubuntu hardy (8.04), which still offered KDE3 was not an option. I had no choice but go shopping for another distro, which was a lot of pain and work at the time. I ended up using Mandriva, where I could use KDE3 and KDE4 simultaneously. So I made my switch to KDE4 gradually, until I had figured out how to get everything working. After some time, I switched back to kubuntu. I just liked the repos and some other things better.
Still, remembering this entire story causes me a lot of pain. Would it really have been sooo difficult for kubuntu to keep supporting KDE3?