The physics engine dynamically modifies game speed depending on how heavy a load it's running. The game will also slow down if something is loaded that is causing problems with the physics engine, like objects spawning half into the terrain where it can't sort itself out.


Your options are:

1: Faster PC all around.
2: Load less complicated vehicles.
3: In the start of a CUSTOM game, you disable a bunch of of the features via the add-ons menu.

Useful keys:
F1: Look at graphs of the engine load.
F2: Visualize physics blocks.

more items more lag

Me and a friend have found that simply restarting your pc can drastically help in some cases. We were Ona game together doing about 20 tps (very slow) and after we both restarted we shot up to 62 tps ( as fast as the game gets aka very smooth).