Registry Cleaners actually don't gain much performance. Even one where you delete 1000+ registry entries and defrag the registry, it's still only like a 5% gain idealistically.
The problem lies in particular if you're lagging in single player or online mode.
If your lagging online:
-Check network connections.
-Check for network intensive programs (torrents) and turn them off or severely limit the upload rate.
If it lags all the time:
-Something might be hogging CPU cycles and not enough is getting to your game
-Your system may have accumulated dust and may be hotter than usual. Cleaning it out will be good.
-In rare cases, you may have an application that's demanding a lot of GPU cycles. Turn it off if this is the case.
-Check drivers (specifically GPU)
-Check in-game settings to see if they got buffed up unexpectically.
Follow these instructions to do a clean boot to determine if some other software is eating up resources:
Link