Basically what Jimbo said.
Usually maprooms are disallowed for a few reasons, but lag isn’t one of them unless the server owner is ignorant (which can happen). Normally its to conserve space as private servers will normally be more restrictive about building space and building limits. Maprooms take up a ton of space and not allowing them will cause players to not want to build structures to go with them.
Another reason is usually to force more conventional travel around the map. In RP and PVP servers this is to ensure players run into each other more frequently. Keeps players from simply teleporting somewhere and then teleporting back.
And some server owners just arbitrarily don’t like fast travel or simply want to decide how its used.
I will say this, anyone saying that maprooms cause lag as a server owner, is raising a red flag. For one, if they are ignorant, it means they don’t quite understand what actually causes lag and issues. And two, if they actually do and are lying… well that has its own issues.
This also applies to any owner/admin stating stacking foundations causes lag. I don’t want to go too off topic here but this is the sort of thing that made the OP even ask the question in the first place is the practices ignorant or deceptive server owners and admins.
To put it simply there is two sorts of ‘lag’. You have client side framerate drop. Buildings that cause this will only do so when said player gets close. This can be dependent on their hardware (weaker hardware is more susceptible), but also can be caused by enough to push the game engine itself in some cases. This is stuff like torch spam or other placeables that have a lot of moving or changing textures.
Then you have server lag, and this can happen to everyone on the server once the area the building is loaded in by anyone. This is caused by a lot of data intensive stuff, like too many thralls moving at once (like a thousand thralls attacking something), excessive amounts of storage, and bodies left in the world when players log out. Now I do want to say that one person probably doesn’t cause a server to lag on their own, but when you have a community of a few dozen to few hundred players, and the server is quite active, every bit of contribution can deteriorate the server over time.
At the end of the day, if your server disallows something because of the reason of ‘lag’ and it doesn’t quite make sense, definitely do your research. Which the OP is doing thankfully.