Hi . i am facing the same problem of FPS degradation while using any sort of TGP/svkhal etc
here is a good topic on cpu and gpu perfs comparaison :
https://forums.eagle.ru/showthread.php?t=157374
unfortunatly, the author didnt specify if he especially tested, the framerate in this situation. i have, well , 40 50 fps in su27 yeah.
but only 18 20fps (fake, with huge frametime delta) with a TV monitor looking at a unit in wood area , at low altitude... and it's a problem i had since flaming cliff. i never had a huge high end computer to be sure it IS possible to get a full time smooth framerate in this sim.
maybe some of you can witness that's possible.
an high end 1080ti gear up with an lastgen i7 MUST NOT show this lack of performance imho.