[Aylan]

Oh right, my mistake. Well if it's server-side it is mainly caused by excess use of callbacks like OnPlayerUpdate and from my experiences, timers and loops. There are cases where the script hasn't being optimised to the point where it has caused lag for players. Of course the main reason is most likely just inefficient code.

Don't forget that there are cases where your hosting service can be pretty bad too which can have a effect on your server. That is just something to take into consideration though.