i logged in to come tell you that your windows paging files must NOT be managed by windows.
Open SYSDM.CPL from start menu and - advanced x2 - System paging file-
It must be at least 4GB - ie 4096MB and range up based on your HDD free space. Roughly 8000-12000 is good for a maximum.
Anyway. This worked to fix games like RDR2 too,

I have had this issue before when either it was windows managed, or the paging file was too small, so the game could start at 160FPS -but by the end, im sitting on 20-40FPS - - Paging file 1000% responsible.

I did also look into launch options etc. and whether i rendered this game at extremely high, or low, it was still not changing, till the paging file was set correctly//