Does your phone run slowly in any other situations? Loading of apps, typing, answering calls. I notice a delay when unlocking my phone if I haven't restarted it in a while, or the cache is backed up a bit.
Easiest thing to try would just be to restart it. Probably a good idea to do that once every few days or so.
The other suggestion would be to go into manage applications, find apps with large caches, and clear them. Tip: while in there, hit menu and select filter. Set it to All. That way, it will show the core apps so that you can clear the cache for them as well.