On Windows 10, changing bit depth and/or sample rates of the active output audio device stopped FPS drops and slowdown on 3 different systems (AMD and NVIDIA graphics cards), specially when driving over mud.
Changing to either 16 bit/44.1KHz, 16 bit/48KHz, 24 bit/44.1Khz and 24 bit/48KHz eventually led to the issue clearing, though it still took trial and error to find out which one was correct.