Having had a flawless MSFS since SU5, being faced with this is somewhat disappointing. With and without DLSS the results on performance are the same. I’ve attempted to troubleshoot with all the obvious tips - empty community folder, reset Nvidia control panel etc… If I am to draw back on graphics quality then that is quite the regression.
I’ve experienced the same, unfortunately. 
I’m experiencing lower frame rates at large airports, down a few frames per second, but they also seem to have far more live traffic than before, and the graphic quality of the scenery and airport equipment seems higher. Although the frame rates are lower the Sim seems smoother, using DX11. I am using the new NVIDIA driver and have tried DX12, but it seemed to introduce more stutters. I’m hoping that the Game Ready driver will help, once released.
Frame rates at large airports have always been lower because there are a lot more objects that need to be rendered. Also, there are 3rd party developer objects and scenery that may not have been optimized yet.
Also, are you using the same graphics settings from SU9 or SU8? I discovered that my SU9 settings made my FPS and graphics worse with SU10. There isn’t any automatic update of settings with the SU10 installation.
FWIW with SU10, I am seeing outside of large airports 50 to 60 FPS. In and around large airports I’m seeing 30 to 40 FPS, acceptable for me. Prior to SU10, large airports were about 25 to 35 FPS.
In the past, there were problems with the scenery in and aroun KATL dropping FPS to about 5 - 15 FPS, clearly unacceptable.
Same – performance is awesome until you’re at the airport.
It doesn’t seem to get mentioned much, but turning AI traffic off and all three Airport Life sliders to from 100 to zero increased FPS from 30 to 60.
I’ll take empty airports over a poor frame rate any day.