When Building in forge after using 1500+ blocks it causes the player and players to go invisible as well as on custom games it also causes the players to go invisible only showing the gamer tag this has been happening since the update and it would be good for it to be patched ASAP as i cant do custom games until its patched.
I was helping someone with a similar issue on the Halo 5: Forge Support Forum. We found that is seems to be in fact a problem with generating lighting rather than the number of objects (though more objects require more lighting). Try to turn bake lights off (and dynamic lights) as much as possible throughout your map to see if there is any improvement.