Hello everyone. Maybe I’m not the first to report this oddity. I am on course with the A320neo, I have engaged the autopilot and I follow the indications of the aerial control on the cruising altitude which increases from time to time. Everything goes well, I arrive at an altitude of 38,000 feet which is the cruise one for my flight. The plane perfectly maintains altitude and speed, everything is ok. I decide to speed up the simulation to get to the moment of the descent. As soon as I set a 4x speed, the plane starts to swing vertically in an uncontrolled way and quickly loses altitude. I reset the simulation speed to 1x but I fell to 32,000 feet and the plane is now unmanageable. This happens every time I am at cruising altitude (38,000 / 40,000 feet) because if I increase the simulation speed below, everything is fine. At this point the doubt is that it is a bug.
AP PIDs are not tuned to run at x4 sim rate.
You are lucky if they are correctly tuned to run at the normal x1 rate 
This is incredible! Any flight simulator provides the ability to speed up time to avoid the boredom of a flat flight. There is the possibility of taking the flight at a predetermined time (for example the descent), but this - inexplicably - completely cancels the flight plan!!! According to the geniuses who have programmed this game, I should endure 15 hours of flight!!! Realism is fine (which MFS2020 certainly does not have), but only a mentally ill person can spend hours staring at a monitor because the simulated plane has to get to the other side of the world.
I feel the same. Even X-Plane, FSX, P3D could manage atleast 8x without any problems, beyond that it depended on the plane.
it depends of the plane, how its tuned
I can do no problem x8 with the Longitude and it’ll stay mostly within 100’ of the assigned altitude
most of the planes are doing well at x4
When some aircraft reach cruise altitude in autopilot, when you speed up sim speed the aircraft begins to oscillate vertically. it pitches up and down to extreme degrees making the plane lose or gain altitude.
this usually happens worse at 4x sim speed. i have observed it on the 737 and the diamond da42.
it looks like the autopilot is trying to control the pitch using trim however it always over corrects in both directions causing massive oscillations, (yes trim assistance is turned off)
would be nice to have this fixed so you can speed up the sim without your plane losing control and crashing.
That’s sorta related to how fast your PC is. Some aircraft are worse than others, but 2020 did it too. When you get oscillation, back down one step and that’s as fast as you can go with Sim Rate.
Yeah, that has been an age old issue since the invention of… accelerated time.