I’ve been wondering about this problem where experience changes depending on the day and/or session.
Are you all using MP and seeing other players? I do always and I’ve been suspecting one possible cause could be displaying other MP.
In effect, the only thing which could vary, unless there is a bug in the code randomly making the experience a misery, is the type of aircraft model displaying for other MP and it is possible in this case 1 or more stock model the simulator is using as a substitution is causing performance problems (bug in the model geometry, too much polygons saturating, an unsupported tag or animation or light, etc…).
Otherwise it is possible 1 or more aircraft MP are flying are port from FSX or FS2020 developed aircraft for which the substitution is causing side effects in performance.
This is also one reason I’m recommending the following you might want to support:
Multiplayer: display other players actual aircraft 3D models and liveries around you