Computer Animations/Games and Video lag?

How old is the computer? I have the same problem with certain games aswell. It may be that your comp. can not handle what the games graphics and things like that. I have lag in videos too but what I do to stop it and make it work perfectly is on the corner there is a number maybe 720,480,360, or 240 (on youtube). If you lower it to 240 it would make it not lag at all, but it'll look crappier but at least you will be able to see whats going on.