Just install my RTX 4090 but very poor FPS - Looking for support

You’ve had many tips now and I think it will take some time for you to digest everything that has been brought up.
Your screenshot shows that with MEDIUM settings your CPU has a frametime of roughly 27ms, while your GPU is sprinting at 6.5 ms.
Regardless of the fps, what this basically means is that your GPU has to wait 4 CPU cycles before it can put out a new frame.

The bottom line is: There is no “one slider fixes it all” solution. You have to find settings which will relieve your CPU and put more strain on your GPU.
That takes time, some experimenting**, and first and foremost some basic understanding of the impact each and every setting has on CPU vs. GPU. To help with that understanding, search the forums for “Settings Guide” or such. A good starting point would be

Unfortunately the very comprehensive settings reports done by CptLucky8 are mostly removed, but here’s a restored post:

Certainly not what you have expected, but that’s how it’s always been: Flight Simulators need a lot of tweaking, and while MSFS has reduced the efforts mainly to in-game settings, it remains a challenge.

Yep, with your 60Hz monitor it would be Vsync On and 50% framerate limit (don’t know the exact description). And while I have always used 30Hz, I would suggest that you include this setting into your experiments as it takes load from your GPU, which in your case is not necessarily beneficial.
The basic concept here is to match the monitor frequency with fps: when you can find settings to achieve stable 60 fps or more, locking Vsync at 100% will give you a smooth experience. If you can’t, go for 50%.

** Take your B737, put it at 2000 ft. over London or New York, program a holding and then change individual settings one after another and take note of the impact.