WHAT FINALLY WORKED FOR ME...
I have made a few posts throughout this thread, from very early on, as I too had the problem of updating to Lion and things slowing way down. I tried many of the suggestions here, but none worked. The one I didn't try, which I suspect may have worked, was a fresh install of Lion.
I decided, however, that if I was going to go through all the work of doing a fresh install, I would take this time to upgrade my boot drive to an SSD hard drive (solid state drive). When I did this, the problem completely went away, and of course, everything was even more lighting fast by using an SSD drive.
After fixing the problem completely with the new SSD drive, my suspeciouns are that it was, as some here have also said, hard drive related. The drive I was using didn't have any updated drivers for lion, so I wasn't able to try that, but by no longer using the drive, it fixed the problem. Here is why I further believe this...I decided to NOT do a fresh install with Lion, just to test if it was the drive or the OS set up. I actually used Carbon Copy Cone to completely clone my old drive to the new SSD drive. In other words, NOTHING changed with my OS' set up, the apps I was using, how they were installed, etc. The ONLY thing that changed, was booting from a different drive, which contained the OS and all my apps, etc.
This isn't obviously the solution for everyone, but I wanted to post to at least share my experience that I do believe it's hard drive/driver related. You should for sure try updating your boot drive's drivers, if new ones are available for your particular drive.
If you have the $$$, I'd HIGHLY recommend upgrading to an SSD drive. It was a pretty easy process using Carbon Copy Clone. It feels like I have a completely new machine! (I did read/write tests and the SSD is 400% faster than my old drive.) My Mac Pro boots in 15 seconds, and apps pop open, even Photoshop, Xcode, etc. It's wonderful! After having lived with the slow Lion problem, to jumping to not only fixing that problem, but a 400% faster drive, I'm in heaven!
Kevin