Began to lag. (Tom Clancy's Rainbow Six: Siege)

Hello! in General such problem: Not played for like a month now gone, and saw that the fps is very unstable, and falls in the area from 40 to 26, although in the past, a month ago on this very same PC with the same settings was stable 90 fps, at first I thought that involved some advanced graphics settings, I decided to look into dzhefors experience but he on the contrary I offered to make the graphics settings higher, but the resolution is generally up to 4K via DSR to raise, why such rigmarole ?