Is it with any aircraft or with a particular aircraft?
Looks like a different issue of what’s being described in here but you might want to check e if you do not have any keybinds assigned by mistake to decrease sim rate.
I assume you are on Xbox. Do you have a 120 Hz TV? If you run with 120 Hz on your Xbox the sim will slow down a lot. I had that issue and switched back to 60 Hz and now I only loose like 2-3 minutes after a 10 hour flight ~
More details here: For Xbox Series X players with 120hz TV