the map is big and it was difficult to optimize, it's possible that it comes from some driver problem, but I can just advise you to try on another computer sorry :/
wouldn't creating load zones help the performance? i suppose you dont need the opposite part of the map loaded at all times, for example. You could divide the maps into regions and only load the ones that are either touching the region that you're currently in or that are within render distance. And in case it breaks any of your scripts, perhaps also the one that has the checkpoint. Really there is no good reason for not doing that.