Just throwing this out there, but I used to have that problem, and it was related to the “Task Manager in the Foreground Problem” discussed here. This issue would effectively drop you out of MR and you would never lock again until you brought a window to the foreground, etc. ANYWAY, in short, have you updated OpenXR? (Not ToolKit) But updated OpenXR from the Microsoft Store? What version you running?
I asked above, but you didn’t reply. What are your thermal temps when the performance suffers? What PSU are you running, and are you overclocking at all?
Also, are you running HAGS on or off?