For those of you who are struggling to understand what my solution was:
Intel processors have the capability to automatically increase their core multiplier. This is widely referred to as “Intel Turbo Boost” and they have Intel Turbo Boost, Intel Turbo Boost 2.0, and Intel Turbo boost MAX 3.0. This allows your computer to automatically increase the frequency of certain cores under certain conditions.
I am NOT advocating disabling Intel Turbo Boost 1/2/3. The solution on my particular PC, with my ASUS motherboard, involved disabling the “ASUS Multicore Enhancement” which was enabled in the BIOS. The ASUS Multicore Enhancement changes the parameters of the way that Intel Turbo Boost operates, and that issue caused severe FPS drop in MSFS to the point where the game was unplayable. Depending on the build of MSFS sometimes the menu would struggle to load. Some MSFS builds were better than others and the effect would be less pronounced.
MSFS is the only program out of about 150 installed on my computer that struggled to deal with the ASUS Multicore Enhancement. I know little about advanced performance implementations like Intel Turbo Boost, so I can’t venture a guess as to why MSFS struggles with this. The CPU load on my 10 core/20 thread CPU would not go beyond 35% with ASUS Multicore Enhancement on, while it will go up to 100% load on certain cores with ASUS Multicore Enhancement off. This is what was causing the immense CPU bottleneck for MSFS and why my game was unusable even on the menus.
I do not have a MSI board for testing but some people have reported that disabling “Enhanced Turbo” on MSI boards will fix this problem. Again, I cannot verify this. As I understand it, any mobo-specific setting that changes how Intel Turbo Boost works will break MSFS. Your motherboard may use a different name for this technology. It is worth the time to investigate it. Again, leaving the regular Intel Turbo Boost enabled is fine, but non-standard Turbo implementations like the ASUS Multicore Enhancement or MSI Enhanced Turbo may be causing these problems.
I am by no means saying that MSFS is well-optimized. It is not. It’s a well-known fact. I am simply suggesting that my game went from “completely unplayable” to “runs pretty well” by changing that one setting in my BIOS. If other people have the same option then I would encourage it. That’s the point of the forums… MSFS tech support has always used the same copy+paste response to my support requests no matter how much detail I provide. So we need to help each other out.