I experienced the same slowdown at first. The performance has picked up. I planned to check if it was re-indexing, but I missed my window. Others may want to check if their disks are taking a hit.
How long after the update would this affect the disks? It's been weeks since I updated but my iMac still seems a little slower than it did before. Lots of beach balls whenever I do pretty much anything I haven't done in a while, including opening System Profiler, System Preferences, etc.
Edit: I did some poking around on the internet and found one of those articles about "how to speed up your Mac." I always take those with a grain of salt and I know you can mess your computer up if you don't know what you're doing, but I'm an adventurous type so I took a gamble.
I had deleted my Safari cache, history, etc previously, but it didn't seem to speed up my Mac, really. So then I deleted basically everything in my Library/Caches folder after reading that it could help with speeding things up, but that didn't do much either. Finally I got up the nerve to delete everything in Library/Preferences except the few things I knew I'd really want to keep (iTunes, Adobe stuff, etc). I accidentally deleted my Mail app preferences, but Time Machine saved the day, because I had a recent backup and restoring the preference files set my Mail app back exactly the way it was before I deleted everything.
So when I finally restarted, it was like having a new Mac. I had to reset all my preferences (startup applications, dock and finder settings, desktop background, etc), but once again my Mac is just as fast as I remember it used to be. So if your Mac's being slow and you're feeling gutsy, what worked for me might work for you. Just a suggestion.
Fair warning: I'm a risk-taking curious little idiot and have messed up my old PCs by doing whatever the internet told me to do. That's one reason I got a Mac, so it could stand up to my abuse.
Proceed with caution.
