Limited by mainthread but mainthread only at 60-80% (UPDATED)

Welcome… but can you please stop to copy-paste your point into multiple topics ( e.g. , e.g. , e.g. ) ! thanks.

Without exact knowledge of your settings, we can not say much. Often the users gave also wrong informations, like the former post in which the user reported 1080p but posted a screenshot about 1440p ( like you, also in different topics, seems contagious ) , so please also share screenshots if possible.

Generally there are existing FAQs and also community HowTo’s you can check, e.g. :

Beside of these it sounds strange that you can not use your CPU in normaly frequency range. Without the normal usage of the turbo, you have of course less performance CPU wise.
Also , just in case, if you use the Dx12-Beta feature, switch back to Dx11 setting. And try MSFS without any external tools or mods, only then we can compare performance and can be sure that nothing is caused from it.