Is this a momentary fps drop when the garage first opens, or does the game only run ar 30 fps whenever you're in the garage? If you play at 4k and it's just a momentary drop, that sounds pretty normal. Menus and replays don't run at a steady 60 fps. But if you play in 1080p and/or the garage never goes over 30 fps, it may be worthwhile to rebuild the PS4's database and see if that improves your performance.