Try replacing the cars with a normal brick, if it doesn’t lag, try rotate the viewport camera position instead of the entire car model.
It seems like the engine is having a difficult time constantly updating each descendant’s position and orientation.
Hi, this is eventually not an issue, roblox does that on purpose, it throttles your viewportframes depending on the amount of insrances in it , and how fast your machine is.
temporary fix:
To get around with the throttling, you need to make sure your graphics are set to max, this will lead to viewportframes to update in realtime (almost).
If this resolved your issue then please let me now by either telling me , and or marking my reply as solved.