Whaat, so even when your not on the Z layer its still there? That is super wierd! That makes me feel like when I finish my game im gonna end up having to keep turning areas on and off to prevent lag >.<.
Yes, the Z layer is there regardless of who's on it. The software can't predict what it's going to need access to and what it's not.
This will not, however, create any lag. The only penalty is that the map has to be loaded into memory and then it stays there. It will have no impact on your players if they're not on it, since they'll never be sent info about those turfs.
Please consider what TheMagicMan is saying!
Only loading a few pieces of a single .dmm at runtime is probably impractical; setting up a system where multiple smaller maps can be loaded and unloaded as needed by the world (it would have to handle the logic for when to do so on its own) is much more practical.
Please consider what TheMagicMan is saying!