I'm not a mapper by any means, but there are two things I can add off the top of my head:
- Water can sometimes be quite demanding.
- Scripts will slow down the game.
Of the two, scripts are much more important. Firstly, the overall number of scripts is relevant for obvious reasons - the more scripts the game has to check, the more it'll slow down. Secondly, not all scrips have to be checked each frame: For many scripts, it's just fine to have the game check each second (or even less frequently) for their completion. That can also have a big impact. For example, some of our scripts for Mordor's Haradrim and Easterling reinforcements are only checked once every second instead of every frame.