Bad performance in single monitor

[Elaphe]

I'm running AC with a 1080 Ti. With Oculus Rift S I keep 80 fps most of the time. I haven't played in my monitor in a long time and yesterday I was shocked to notice that I had frequent frame drops from 60 to 50 and even 40 in several of the tracks I normally use, and that is running the game at 1080p! How is that possible? It's obvious that a single screen at 1080p/60hz should be no problem at all with my specs. I'm using CPS and SOL. Could it be some options that are incorrectly set or killing the performance but only affect the single monitor setup and get disabled when in VR? That could explain the differences in performance. Any idea what to check out? Thank you.

 

[Stereo]

The majority of ExtraFX only works on single screens. That'd be the first place to look.

 

[Elaphe]

I have checked the ExtraFX tab and I see it comes disabled by default, where it says "Extension", the "Active" field is unchecked. If I activate it, the peformance is perfect, and the graphics looks better, like with antialiasing. If I leave in unchecked (as it was) the graphics look too sharp and with jagged edges and the performance is much worse. I don't understand the logic of this.