Hi, yes, your computer is not bad, except that x-plane does not work very well with AMD GPUs. You should expect improvements with the coming updates during the next year.
That 20fps limitation has always been there, because it's bound to the way x-plane has been working for years (at least since x-plane 9).
In the meanwhile, make sure all your drivers are up to date, and delete (or rename or move for backup) your ...\x-plane 11\Output\Preferences folder. Your settings will be re-initialised but you might observe framerate improvements. If no change, you could restore your backup.
Guillaume AMELINE (not affiliated with Laminar Research)