With the arrival from The Valley DLC, the game had undergone an update which impacted the performance of certain configurations. The development team lets us know via its forum that a public beta is available on Steam to correct the situation.
“Hello everyone, we are announcing the arrival of a public beta to test a fix for the performance issues that have been noticed since the release of MudRunner's free DLC, The Valley. This beta will last until Monday, February 19. In this public beta, you will be able to play multiplayer with players who are not in the beta. This latest beta is available as a public branch and can be viewed by doing the following:
1. Locate Spintires: MudRunner in your Steam library
2. Click Properties, then Beta
3. Select public_beta from the drop-down menu
Once done, you will be signed up for the public performance beta. This should fix the performance issues you've noticed since The Valley released. If you continue to experience issues in this beta, please provide your logs (which can be found here: C: \ Users \ Name \ AppData \ Roaming \ SpinTires MudRunner. They are generated by holding SHIFT during game launch) and your mod list.
If the beta is worse for you than the current version, please go back to the base game, and give us your logs and mod list (as detailed above) on the game forum HERE."