fps drops after changing car

Hi, I have a problem with beamng. When I spawn a car, let's say a BMW 530i, I have 60fps. For example, when I change the configuration to 540i, the fps drop to 30-40fps. When I change it back to 530i, the fps also drop to 30-40fps. When I change it back to 540i, it's 60fps and it's like this over and over again with different cars. It's the same with tuning. There's a car that has 60fps. I change the exhaust, and after loading, the fps drop to 30-40. When I make the exhaust the way it was, the fps are still 30-40 and so on and so forth. When I change between two exhausts, it suddenly returns to 60fps. Please help.