System Pannel lite allows you to monitor CPU usage. You can see if your processors are running at high speed a lot.
If an app is indeed not working properly this will cause the unit to spend more cpu time on that app and cause your device to become sluggish as well as drain the battery faster.
But as the other posters said, usually if a device works fine, then suddenly becomes slow its either a recently installed app or an update to an app that didnt take to well.