Okay I see your error with your first trigger it's common sense.
If you place too many units wc3 starts to lag, right?
Well, not only does the graphics engine lag, but with this rapid commanding, that lags, too, causing command delay, which is when you tell something to go somewhere and it begins to do the action about 2 seconds after the action was ordered. This is caused by too many units. This is more commonly seen in roleplays when the player is handling cities and armies.
What I see to do is to make the 4 regions of the map. Have every 5 seconds 2 new zombies created in each sector, or region. This prevents too much lag. Order them to randomly patrol their sector. Patrolling is basically an attack-move when they are going around the map randomly.
This is quite complicated so I won't give you the trigger for it.
Now I have a question.
Do the players get to select their hero? Or is there only one hero [for each player] that is already on the map in the world editor?