The OS as a whole seems a lot slower with the new 2.2 update. I've experienced similar lag issues in screen transition and app loading. I think the reason is that the new update is a little heavy for our outdated phones. Since I've had the phone, I've rooted it and run custom roms. The benefit of this is that I can delete any bloatware apps that I know I won't use and have a much slimmed down version of the official update. Not saying it's a solution but it is a decent alternative.
Also if you root you can always downgrade back to the 1.6 OS before you received the upgrade.