if you use these magnifying glass right top corner, you get your answer 
… for all of your mentioned issues ( fps in-flight, fps in menu )
There are existing thread(s) in which you can join and participate.
But beware : you use a mod and if you would read the REAME first, you possible find out that this category is for “no mods installed”.
Example:
https://forums.flightsimulator.com/search?expanded=true&q=performance%201.13.1%20order%3Alatest
( or somewhat with spikes, or or… all your ‘own’ keyword can you use )
Or more easy: scroll throw the category in which you created your topic
PS.: and in case you read a bit, you possible find also out that users with 16GIG needs a bit attention