Minefield modules drop performance terribly, with 4 of them on a completely empty map (0 AI, 0 objects), it drops Dedicated Server FPS from general 50 (everything empty) down to 30, with about 6 of them it goes down below 10. And that's on a beefy machine.
By the looks of it, the module does some loop checking constantly if something approaches it, and there are no visible mines at all, you just get blown up from something magical in the air. The module should function so it puts down mines on init around that location, that would get rid of both the horrible performance and invisible mines.