Onyx has a well-sized load of disk and OS cleansing utilities that are easy to run, and generally quite effective. After cleaning heavily-loaded caches and running maintenance scripts, my system has usually shown a noticeable improvement in how it runs. After tasks like a full system optimization, restarting takes a little longer than usual, but afterward the system is running a good deal zippier than it did before. Of course, the fact that Onyx is free and continually updated makes it well worth the download and testing time. If your system starts lagging, I'd definitely recommend giving some of Onyx's tools a shot.