[these are what I recommend. These things are included in the guide]

1. install Luuks preformance mod[www.beamng.com]

2. Stop unnecessary background tasks

3. Remove unnecessary/all mods

A big issue is your CPU or GPU. Here is what they improve in BeamNG.

CPU is a main thing in beamNG. Having a good CPU will prevent lag spikes when crashing or driving. This is because with good CPU, it is more able to run multiple calculations in beamNG

GPU is a 2nd thing. My GPU is crap but I still run BeamNG pretty good. Your GPU is what runs through the calculations. So if your CPU can hold them and manage them, then your mostly good. A bad GPU may limit the amount you can do of something. (Like amount of cars)
[this is not accurate to other games, and is only accurate here because its logic in the game apply's like this]
What you should do
1. Have a good CPU
2. Don't focus too much on GPU

If your CPU is kinda bad or is just being used a a lot. This could be an easy fix.

Using task manager to close unnecessary processes.
WARNING! DO NOT CLOSE PROCESSES THAT ARE REQUIRED TO MAKE YOUR COMPUTER RUN

Things like your browser, or your recording software running in the background can cause lag

Self explanatory, try disabling all your mods. (recommended) Or, try moving them out of the mods folder temporary. This can fix mods that are broken/unstable

Alternate fix (recommended)
You can try downloading performance mods. Like "Beta - Luuks Performance Mod from BeamNG"[www.beamng.com]
or UPP[www.beamng.com]

Sometimes, reinstalling the game can fix issues. This has worked for me.

You can also try waiting for an update that fixes this issue.

If all else fails... turn down your graphic settings. We know nobody wants to do this. But this may be your only hope....

And if that fails.. Don't worry I got a final solution. If nothing else works, cry yourself to sleep.. This will relief all stress. Then, you can peacefully uninstall the game...

Hope this helped