Welcome to the forums! I would try a couple of things first.
1. Are you on BES? If so, is the phone still running Enterprise Activation? The phone is very slow while it's doing the Enterprise activation. If this is the case, set it down and let it finish the EA.
2. Remove the battery for about 10 secs and reboot the phone.
3. If you are using the zen themes, or dimension themes they are memory/cpu intensive for the older BB's. After a reboot each time you access a new menu item it has to build that menu item. After that it stores it in memory so that next time you access that same menu it brings it right up. After you access all the menus the phone will be back to normal. However, when you reboot again you'll have the same problem until you get all the menus loaded again. You'll notice that the first time you get an incoming call after a reboot you will miss the call. I would call myself once so that the BB will build that screen then it's normal after that. The initial lag isn't so bad on the older themes.
4. If none of these solutions work then it could be third party apps. Maybe one is dragging it down.
5. Lastly, try wiping the handheld with JL_CMDER (easily found with a search of the forums) and then load the OS again through Application Loader in Desktop Manager. You can load the same OS or a different one if you like.