I have a game where the map in generated by a script, it has a lot of bricks though. It is very laggy when i run it, to the point where i can't use tools/weapons. Ive seen plenty of other games that have tons of bricks, yet they run smoothly. For example the Quarry, it somehow generates maybe over a thousand bricks for mining and yet the lag does not seem to affect it very bad.
If they can't see it, don't generate it. There's simply no point in making things which may even never get interacted with by the player. It keeps the RAM free, it makes sure that there's less for the GPU to worry about when rendering and working with its own occlusion (Note: Roblox occlusion is bad), and it frees up the CPU at the initial generation by reducing how much stuff is being done ahead of time by moving stuff to the run-time.