Fluctuations in Processing Speed

Exporting Prores source as H265 using V6 with no enhancements I’m averaging 18fps. Using V5.5 I’m averaging between 85 and 200FPS, however using V5.5 however it hangs at 5%. However I’ve also experienced slow speeds with 5.5. Can somebody please help me understand what informs the fluctuations I’m seeing in processing speed?

Using M1 Studio