Yes, I finally solved the problem. The problem was not actually caused by the cars or the code. Of course, using static instead of skeletal mesh gave me 3-5 more fps, but there were two important things that helped me reduce the drops. I solved this by typing stat Game in the console (https://docs .unrealengine.com/4.27/en-US/TestingAndOptimization/PerformanceAndProfiling/CPU/)
I previously examined the events of the cars with the session frontend and did not see any excessive MS, so I resorted to this.
I left the game open for 5-10 minutes (editor) and saw that nav tick and world tick reached excessive MS values after a while, then I realized what I needed to solve.
1- I turned off the affect navmesh feature in cars(because I had people walking around and using navmesh)
2- From project settings, I increased the Time Between Purging Pending Kill Objects value in engine - garbage collection from 60 to 600.
3-Actually, the two steps above were enough, but rather than fps drop, I opened nanites to some high poly meshes on the map (even though I gave them lod), which gave me a few more fps.
I hope it will be useful to others as well 