I'm having the same problem. Someone upthread said this is a known issue and there's a JIRA on it. While that's not particularly helpfull in solving the problem, at least I know I can stop fiddling with things trying to fix it so I'm glad I found this thread. I've been on SL for 6 years and been through some crazy issues but its never, ever been this bad. It's a total lag and crash fest for me on both Firestorm and the SL latest viewer. Whatever this is I hope they fix it soon because it's hardly worth logging in.
Just wanted to let you know you're not alone.