The thing is moving from Snow Leopard to Lion should not have caused a slow-down. Period. If it was running fine on SL, it should run fine on Lion. The only reason Lion would be slow in this case is if something else/new was added and is in use with Lion, or if your hard drive is now short on space.
If nothing else changed other than installing Lion, then that is where you want to spend your efforts to fix.
I would first try Repair Permissions with Disk Utility. I always do that with a new Os that was installed on top of an old one. Usually I get a one-time repair of a bunch of items.
Test to see if the problem went away.
Next I would try a Safe Boot, and then just restart normally instead of logging in.
Test to see if the problem went away.
Please report back here with whatever you did, and post the results.
Hope this helps.