How do you swap the model? What's the command?
Most of the times when someone asks why the game laggs with mv3d its because of to many events on a map. You can have thousands of models if they are not to high poly on a single map without any laggs. The most time I ran into performance issues then it was cuz of way to many events. Reduce the amount and try to split your bigger maps into smaller ones.
use the model(filename.obj) function for that you can swap to any model in your folder back and forth