game suffers heavaly from optimisation in its current state. If I where to guess this has something to do with the way the walls are drawn. by the looks of it they are a bunch of tightly packed vertical lines. while there is nothing wrong with this on its own, every time 2 of these lines overlap the pixels in between need to be written to twice as often. obviously this is just a guess as I do not have access to the games source code. If this is the case I would advise looking into some form of culling.