Ranked in these QuestionsQuestion Ranking
Pro Easy to re-install the OS
If things have gotten out of hand and extremely slow, it may be best to reformat the whole of the OS to a new clean install. This should give close to factory performance similar to when the device was new, allowing for a fresh clean start. It is of course recommended to back up any important files or media (if not already backed up to the cloud) to an external drive as everything will be wiped from the device when doing a re-install.
Pro Built into the OS
Easy to get to by pressing Command + R at boot.