Often going into the app manager and clearing the cache of a select app will let you know which app is causing issue . if the phone doesnt lag when that apps cache has been cleared and then lags after the app is used this is a good sign theres something wrong with the app . You can also stop the apps. most people dont know that an app stays active and running until you close the app. Just going out of the app does not shut it down it remains active in the background until you actually close the app . Hope this helps