Further to the above, I have included a video that shows the issue here (
https://streamable.com/8bkenu). The good news is that it is only temporary, and goes soon after loading the save.
Although it persists through system restarts (so whatever is happening is in the save file), it seems to go after waiting or moving on the map after about 30 seconds. Is there a massive battle in the background which is being calculated or something? Would this even affect the FPS? I dont know, but it is weird.