I have an i7 8700, 32gb RAM and a GTX 1080 Ti, fine specs for the sim, and it used to run at about 35-40 fps buttery smooth (for a sim) on (1080p) ULTRA with everything maxed out before world update 3. I noticed that one of my CPU cores was usually pegged at 100% and the GPU sitting at about 50-70%, the fps reader showed that I was Main thread limited, no problem, totally normal for FS2020.
BUT after the update, the sim dropped to 20fps in areas and planes I used to get 35fps or higher (I tested this objectively) and upon further inspection, I was mainthread limited BUT my CPU core was now maxing out at about 75%, GPU dropped to about 30-40%. I made sure I was on windows high performance mode etc etc but still no dice. Have a look, I used to get over 30fps here with this exact aircraft, now 20:
The issue here is the difference between 20 and 30 fps, IMO really makes it a bit annoying and sometimes borderline unplayable