game is lagging, low fps

Please note that the post you replied to is from 2013.
Sometimes it's better to create a new thread

Give some more details:
- Full machine hardware specifications
- What settings/resolution you are playing at
- What level you get that on? (or if any level)
- Is the game using your Dedicated Graphics? (check in Help > Performance in-game)
- Playing with the laptop charger plugged in?

 

Surface Pro 4 is not exactly a gaming machine.
It only has an Integrated Graphics, which by itself is not suitable for more complex 3D games.
Even worse if you try to run the game at the Surface screen's native resolution (1800p something).

You can try lowering all the graphical settings, and using a lower resolution.
More than that only an actual PC near/over the recommended requirements would help.

 

You need to give much more information, such as:
- Full specs
- Resolution and graphical settings you play at
- If that happens on any level or one in particular
- If you have mods installed
- The framerate you get

 

Check that the game is using the GTX960M and not the Integrated Graphics, by going in MainMenu > Help > Performance and see what card appears in the 'Video' panel.
If your 'Intel HD Graphics' (Integrated) appears, you need to configure your Nvidia drivers then: https://wiki.beamng.com/Dedicated_Graphics
Also check your power-plan settings, they can prevent your GTX960M from being used if they are set to preserve battery.