thanks for the answer but still same 
,I changed the car to static mesh and tried again, but again, there is only the spline tracking code on the same car, nothing else. In fact, when I remove the cars, the fps is always constant between 95-100, but when it becomes a car, after 4-5 minutes, sometimes it drops to 50, sometimes it drops to 30, but it goes back up to 95.
(I assign the spline reference to the car I spawned inside an actor called spline path.)
