I've played the game for a few days, and the performance has been stellar. No issues, everything working perfectly on near maxed out settings.
When I tried playing it yesterday I suddenly had absolutely appalling performance, even at the main menu screen with the rats. My CPU usage is minimal during this time. My GPU usage is 100%. I tried lowering all settings to absolute minimum but I'm still at 100% GPU usage and the game is running like crap.
Other games are working fine.