I think it's been 3 years since I last read a issue like this happening. 1. What mods do you have installed and or ever have had installed. 2. Are you sure your on version 21.4. 3. Have you tried playing in safe mode. 4. have you cleared cashe and last have you tried doing a fresh install https://documentation.beamng.com/support/support_tools/clean_install.html Also a video of the issue is needed since your issue seems vague. Do you mean like coming to a complete stop or just a normal slow down. If it's a reduction in some speed that is normal if it's a full stop your having a bug that was fixed and may have resurfaced for you for some reason. Edit: Have you also tried using anything other than the default mode.