A couple of suggestions:
– Within the Windows control panel, under power options, ensure that ‘high performance plan’ is selected
– If this is indeed selected and the slowdown continues, run ‘resmon’ from the start menu and under the CPU tab,
let me know % Maximum Frequency during slow periods.
I wonder if perhaps your systems are intentionally throttling the CPU due to high temperature load.
If this be the case, use the free tool ‘SpeedFan’ to report in more detail what is happening with the CPU temperature.