The first time you use a model, be it for a unit or a spell, the map lags. If you preplace all the models you can be using, it will cause a longer load time but then no visible lag when the units come to being in the map for the first time.
I've never had any lag issues whatsoever with my multiboard. You might want to post your code, Dread the Undead.
Triggers that deal with everything in the map can cause lag if there are lots of units or objects that it's dealing with. Think about how many of different kinds of objects you have, and if you can cut back on any of them. Furthermore, think about how to best deal with only the units you need to deal with. For example, try to avoid things like "Pick Every Unit in Playable Map Area".
Generally, a trigger itself won't cause lag unless you put it together in such a way that it's doing more than it should. If you posted some of the triggers that you're worried about, we could offer some input about them.