This is a known issue with pro tools 10 - it bugs the hell out of me on a daily basis - all the timesaving features like clip gain that came with PT10 wiped out by this. I do a lot of editing on most of my sessions, and open playlists are needed a lot of the time - shouldn't be a problem really. Oh, and if you want to see your system slow down even more , try turning Quickpunch on when you've got a few playlists open!
Apparently it doesn't happen on every system (it's especially prevalent on Windows 7 64 bit machines, but has been reported on 32 bit windows as well as Macs) It definitely arrived on my system with the version 10 rewrite. I'm running an i7 processor, 12 GIG ram, have set all the recommended settings and Windows tweaks, and believe me I've tried every possible combination of everything to try to get rid of this issue to no avail.
Apparently AVID are aware of it and are working on it, but that's been a few months now - hoping the new update adresses this.!!