Yesterday i started playing BeamMP again, but all of a sudden it was very laggy.
I used to have 50 fps on high graphics on Utah with 10 players, but now i have 15 with even very low graphics.
If it has been a while since you played the game is probably recaching all of the resources on the map, the lag in this case is to be expected as you drive around. It should subside once the game has finished recaching the map. This will occur every time you launch a map for the first time.
Try play for a while longer and let us know if the game smooths out. Also, keep in mind that, depending on how long it has been, there have been several graphical updates to BeamNG drive, these new settings like dynamic reflections might need to be disabled if your machine is under powered.