Looks to be a stuck Nvidia VSync.
I had it off everywhere (really, everywhere), but now when I forced it to Off in both General Nvidia Control Panel setting as well as for DCS.exe alone, it seemed to help, but I only had a minute to test, so will confirm later.
Weird thing was, that even this setting didn't seem to work, until I've Alt-Tabed into Task Manager and back while I was flying (and looking at the direction that gave me 30 FPS) to finally disable the VSync.
I was Alt-Tabing with previous settings as well, but without any success.
Can't wrap my head around it just yet.