Practically the same thing described in https://discuss.moodlebox.net/d/112-moodlebox-3-1-0-poor-moodle-benchmark-score happened again today!
This time I started with anew about 10 days ago. MoodleBox current, 3.1.0 written to SanDisk 16 GB, identical to the one the first time. Did many things on the software front: system upgrades, Moodle upgrade, MoodleBox plug-in upgrade, uploaded courses, added Themes, added a few plug-ins, ..
BTW, Moodle Benchmark gave me excellent scores, 150 (Pi 3B) and better (Pi 3B+).
Today "suddenly" the login took a very long time, 10-15 seconds. The Benchmark score is now down to above 4000!
There was one hardware issue though. I connected an external USB3 drive to the MB and it immediately gave the low voltage warning. The funny thing is that the warning did not vanish after I removed the drive. It remained until I rebooted.
May be there is a connection. But I was less careful about low voltage for years without this happening. Have you experienced similar issues?