Just noticed I used curve 200 steps (something I tested initially I guess) but also as I see it doesn't really affect the amount of details, it just makes it more smooth on corners. And yes, I can confirm now that when I used spline segment IDs 1 (so no curb lowering for cars) it saves pretty much instantly, but I after I change some segments to 2 save and scene behavior becomes terrible...almost unusable. So takes 6 mins to save a 10MB file.