I’ve had this simulator for almost 2 years already and the performance for me is still atrocious. The new PMDG 737 which runs amazing for everybody runs like ■■■■ for me. It’s always 20-30fps in any airliner, and that seems like the most I can get after messing around with settings continuously. I know exactly what the problem is, it’s most likely due to my old CPU since the simulator is very CPU heavy for me and the performance monitor constantly says “Limited by mainthread”. Even if I change the settings to reduce the CPU load and move the load onto the GPU, the FPS remains the same. Both CPU and GPU usage always hover around 30-50% and the CPU usage never gets higher. If I move the load onto the GPU (changing render scaling), I’m able to max it out at 90% GPU usage, but with a reduction in FPS so I go up to a point where the FPS does not reduce (around 150 render scaling). Even with higher GPU utilization and lowering CPU-heavy settings, the FPS does not increase and stays the same.
I enjoy flying airliners but I’m so tired of getting 20fps on the ground and everything is very stuttery. SU5 (performance update) did nothing to help for me. I don’t understand why the sim refuses to utilize my CPU more, it obviously has headroom according to task manager. All I want is for it to say “Limited by GPU” like everyone else and get decent performance. This issue has been bugging me since day 1 of MSFS and the PMDG 737 (which performs well for everyone) is like the last straw.
Specs:
Intel Core i7-6700 @3.4GHz
NVIDIA GeForce RTX 2060
32GB DDR4 2133MHz RAM
Windows 10 latest version
Sim installed on SSD
My PC specs are not as good as yours, and I have no issues like this.
This is link is probably the one article that helped the most for me.
You can also look in the vid at his Nvidia settings and set yours likewise.
Didn’t work. Messed around with the nvidia control panel many times, I’ve basically tried everything there is. And I’m talking about airliners, not small GA planes, those run perfectly.
That’s too bad, airliners are all I fly, I rarely get in the GA stuff.
Silky smooth operation anywhere.
I only have 16gb ram, so I created a custom page file.
I’m not sure that would help much with your 32, but have you tried a custom size one?
Google “msfs limited by main thread”, you’ll find my exact problem 
I have a Ryzen 5 2600 - close match to your CPU and a 2060 12GB (oddball I know) and 16GB RAM less than you.
I’m usually at 45-60 in the air (lower end over PG/heavy autogen cities), 20s-30s at major airports.
The key for me was 100 percent scaling at 1080. I’ve played around with LOD values but I finally kept everything around 100. The only two things I won’t compromise on is Clouds to High and Glass Cockpit Refresh Medium.
What I did way way back in August '20 launch days was use the Low end Global Graphics setting in the sim, and tweaked from there.
I only recently switched from an RX580 8GB to the Nvidia, but the FPS numbers I had above were pretty close with that GPU.
What I have found with Nvidia is DO NOT use GeForce Experience. Do not even install it. Leave the 3D settings at default. Everything else falls into place.
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Try terrain LOD at 100 or below, this slider has a major impact in CPU-limited scenarios.
I have a 7700K overclocked to 5GHz and I can’t push terrain LOD beyond 150 (for airliners) and 250 for VFR flights.
Hope you find a good compromise 
You are not alone. My 10700k is not the greatest CPU but it will struggle while on the ground with an Airliner @200 LOD. FPS is over 30 but it stutters while panning. Mine jumps all between mainthread and gpu.
Mainthread is heavily influenced by the Traffic settings. I have all my traffic settings set to Zero, but lower is better.
Try not to run any other applications while you are running the sim.
If you have multiple monitors, disable the other monitors in the Nvidia control planel (deselect them in multiple displays tab). You don’t need any NCP settings for the sim, and you should Restore defaults in the NCP, especially after installing new Nvidia drivers.
Do not run any external FPS monitor programs unless you are testing.
I hope this helps.
My terrain LOD is currently at 10 and everything is ugly yet I have not seen my fps go past 30. Leaving it on 100 gives me 15fps so yeah this setting had the most CPU impact for me. Thanks for the tip but I have already done this a while ago.
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Already knew this, thanks anyway.
Make sure you have Hyperthreading ON on your CPU, I have a i7 6700k and MSFS run nicely for me. 6700 CPUs have 4 cores / 8 threads and the sim makes good use of them all.
Moreover, is your DDR4 ram indeed rated at 2133 Mhz?
It could be factory rated at higher speed, if this is the case you should set XMP on in the motherboard bios.
Hope this helps
Hyperthreading has been on and yes, my ram is indeed 2133MHz. All my cores still have additional headroom and the sim refuses to utilize them.
One other thing to try, turn off the 3D terrain in the G1000.
I never even mentioned Garmin-equipped aircraft lol, I said airliners only.
