I use current openbeta 2.7, but even with non-beta I had the same problem during my roughly 2-months of playing DCS. I use Su-25T, but the smoke seems to be the issue. Granted, my RX570 8GB is oldish, but normally I have 20-30 FPS which is OK for me. I lowered many details, but I don't know what makes the smoke less dense.
In my case when the smoke is too much, the game freezes with Steam overlay FPS indicator showing anything from 1-5 FPS. The sound goes on, if I was shooting guns, their sound is stuck. I can go to task bar and end the application, in which case it actually switches to DCS with its dialog whether I want to quit and this works. But nothing else does. Except for quitting and sound, the game is totally frozen. GPU runs around 100% at that time.
It always happens with smoke or some similar particle effect in the screen. Happens when I switch to ground unit that is smoking, or when bombs explode, when my plane is on flame, etc.