basic principles for make a mod without lag for players ,What should I avoid?

basic principles for make a mod without lag for players ,What should I avoid?
what can, what can not?

Dont use more than 32 moving brushmodels (dont know if this includes players or not)
Dont allow more than 32 players to connect
Dont setcvar them during the game a lot (max 1 cvar per player per frame)
Dont bullettrace every frame for every player (everything less than that should be fine)
Dont do extensive mysql queries (if you are using libcod) during the game

I know from some Zombots servers, that they are lagging, because the Zombies change every 0.5 secounds (or fewer) their models.
And there are 5 (or more) models for a zombie per secound (so that looks like an animation) and there are exist 25 zombies.
So that means, that every 3 secounds have changed 125 models (or fewer, I don't know, but very much).