I’m having this issue too, mainly on long haul flight longer than 3 hours.
I have this with all airliners with mods and without.
None of the above helps.
Specs
Win11 stable
Rtx3090
10700k
32 Gb fast ram
1TB m2 Samsung 980
Settings:
All ultra
4K
Vsync locked at 30…and it never goes below that, always super smooth
Online settings: all ON (traffic and live weather) why shall I fly without those? Long haul without those is booooooring 
After 3 hours, my frames drop to 15-10 FPS, dev mode shows manipulators and main thread being the bottlenecks with responses higher than 130ms
@Jummivana can you please make sure this thread is visible to devs and can you confirm this is being looked at? Thanks