All I can add is that my system from one session to another is drastically different without any changes to my drivers or anything else.
So, I think we are at the mercy of how the MS servers are behaving at any given time and if in Multiplayer Mode, what planes, mods or Liveries other players are using.
Yes, we have determined that when Multiplayer mode is on, it can severely affect your performance.
When I say we, I fly every Thursday on the 19:00 Calendar Event, and we have seen positive evidence that a single player can affect all our performances.
When a specific player left the game, our performance was fine and when he returned, it would go back to stutter city.
That one player started removing items from his Community Folder to try to narrow it down and in the end we couldn’t. But it was conclusive that when he left the game, the performance for all of us increased.
We ended our scheduled flight and never found out which mod in his system was affecting us all, but we all knew that one single users was affecting us all. And he knew it and as mentioned he was trying to determine the cause.
So your mileage can vary extremely without making any changes in your system from day to day. And we really do not know what the root cause is.
If there are any non-believers of what I am saying, feel free to join us Thursday and ask about it. We fly and have open discussions about MSFS.