I have the same problem.
Gizmo64 need 3-10FPS it jump in an intervall of 5 seconds up and down.
I look in my Taskmanager but there are no problems with perfomance. My CPU run with 85% and my Videocard with 9% and Ram with 66%.
On ground i have 20-30 FPS but in Flight the fps drops to 5-20FPS. When i set the Video settings to middle i have 15-20 FPS but i have stuttering.
Same with Video setting low. I tryed to deactivate all plugins except :Better pushback, Avitab , Xpuipc, Gizmo64, Airplane plugins, but that dont help.
When i fly TBM 900 there no problems with Gizmo64 and i have normal performance.