I play at 4K Ultra settings, 3090 I get similar frames at busy airports, and around 40-45 when in the air with the CRJ, slight drops when high clouds around. Sadly our 3090’s are not being used as well as they could be.
Dont know how long you have been simming but FPS has always been an issue in Flight Sims. You are very lucky to get those frames. As much as we all would love a solid 60fps its not always going to happen in Flight sims, you can have a good experience with 25 fps, or tone down your settings. Also not all add-on airports are the same, some devs optimise better than others.