Before you reinstall, why not do a couple of easy isolation exercises - the network, other processes or your user account could be the issues and none get resolved with a reinstall.
Lion has obviously made Preview app do more things. It tracks open files so they get opened again at the next launch, it has smarter detection of data, a new loupe - your mac might just have more to do and this could be the proverbial "straw that has broken the camel's back"
Rather than just reinstall - you might poke a bit to see what it really going on. Are there other background processes? Is it still indexing the hard drive?
The order here might help, but feel free to do whatever makes sense: (you'll run your preview test after each of these steps)
- Turn off the network (all of them if needed)
- Log out and back in
- Make a new user
- Safe boot
There are some great but exhaustive procedures on isolating a software issue on apples support sites, but these are a few that might fast track you in ruling out this one issue.
If preview is slow with nothing else going on - you have a good chance at figuring out why. Best luck in chasing down the culprit.