I speak now for OFP, not ArmA, but I suspect it's much the same.
Triggers poll (check themselves) every 0.5 seconds or so if the condition field is "this". Otherwise, they poll every frame. Triggers themselves cause very little lag: in other words it is possible to have a mission with a very large number of triggers. What causes the problem is loons and wagons interacting with triggers, particularly if they are near the trigger boundary.
Triggers are very powerful and sometimes wrongfully neglected by mission makers who are coders by hobby and therefore used to scripts. In general you should start with triggers.
The thing to watch for in scripts is loops. Fast loops eat resources. I can't remember the fastest possible loop but there's no point in writing ~0.000001, it will still check every 0.05 or whatever the limit is.
The place where you can really get a benefit is when you really need something to be checked only every 5s, say. In that case you're much better off with a script. In this particular case, I'd leave it with the triggers unless you have a large mission, in which case use the script.