Not sure if its just me but, On days 1-5 I normally would get 40-60 FPS but as soon as the boss fight comes about I get about 8-10FPS.
Computer Specs are
2x Xeon Platinum 8176 @2.1GHz it does boost to 3.8GHz on a couple of cores
AMD RX 580 8GB
128GB DDR4 ECC
I do also run the game with Linux under Proton.