Before any thinks so, I am NOT a freemodeler, I'm just asking this question because I'm thinking about working on a huge project.
Is there any default ROBLOX settings that causes lag?
Many bricks in one place can cause lag.
Many scripts running at the same time causes lag.
Basically, many things running at the same time causes lag. So just try to cut down with this. Spread out your bricks, make your scripts simple, etc.