Update: I decided to try a privacy/security utility called Blackbird, on the suspicion that push notifications are what is causing the hourly-ish service logins that appear to cause - or at least, initiate - the priority/FPS problem. Since I installed it I have not had the frequent service logins and have not had the FPS drops.
I don't think the problem is technically FIXED, but as far as I can tell the problem has stopped occurring, with the negative consequences (that I know of, so far):
- The Windows Notification/Action center applet no longer works. This was only ever an annoyance to me anyway as it was never anything but ads from MS.
- Windows Update push notifications and automatic update installations from MS probably won't work any more. tbh these were pretty annoying anyway, especially the kind that would nag you every few minutes to reboot to apply the change. Manual updating is OK with me.
I'm not exactly recommending Blackbird because it's like a giant sledgehammer when I really wanted to solve a very specific problem but at least the problem seems to be solved, or at least worked around? Hopefully without breaking anything actually important.
Link to Blackbird: https://www.getblackbird.net/