How old is it? probably the the thermal paste dried up, causing heat problems and making the game run like poo.
If you don't know anything, and are afraid of messing things up. Ask some technician to check it for you.
That's probably it then, use a software to monitor the temps while you play, if it gets too hot, change the thermal paste, it's pretty easy and cheap unless you're braindead.
Youtube has all the info you need.