It just warns you that your computer is too weak to run x-plane in a satisfying way. You should at least reduce your x-plane graphic settings.
More details here :
Found in the comment section of the previous link :
Ben Supnik says:
October 12, 2017 at 10:06 pm
YES! To item 3. If your FPS is below 20 fps, time in the sim is slowed down so that the simulation can remain fluid and not blow up.
If you load us up with rendering or have a slow computer we only have a few bad choices:
1. Turn down graphics for you until things get fast. Old timers will remember Austin rolling in the fog and cutting vis to hit 20 fps.
2. Leave fps low and let the flight model disintegrate into a puddle of goo. Not great...and the bug will manifest in a million other bug reports. "The plane rolls too much." "The plane rolls too little." "The brakes don't work."
3. Run the sim slower than real-time.
This still confuses people - Austin has that nasty message on screen when you get below 20 fps in an attempt to explain to users that they should cut options to run faster - otherwise they email him and say "flying from KBOS to KJFK took 2 hours at 8 fps."
I didn't know if you can disable this message though.