Gizmo is very likely to be the cause.
Early in the release we did some deep digging into the issue.
There is code inside gizmo that calls sleep(0)... (the reasons for which is beyond the scope of this reply)
Windows, at its discretion, can sleep for upto 16ms/whatever it needs to service the rest of the OS/apps. This can show up as dropped frames or massive perf chugs.
There is a command line option for xplane to leave some cores alone so the os can service itself on those. It helps but the real solution is to fix gizmo..
I have been working on a gizmo update that is based on the three? Year old version that X-Aviation ships.
It contains bug fixes for the most prominent issues (including this one) but is mostly an LTS update. It will also have Apple silicon support.
I hope we can bring it to you all as an update soon.