Scripts are not really a big lag creator, unless you have fast loops as Artak says. Triggers check themselves about every 0.5s so there is rarely much point in having a loop much faster than this. However, if you use
@ condition
a lot in your scripts that could create lag.
Waypoints are more of a problem than you might think. Triggers can be a problem, but you need a more than 20. Aircraft are the real killer when it comes to lag, followed by vehicles. Vehicle type can be important - if you have 7 different types of tank in your mission it will run slower than if you only have 2.