okanta Reminded me of my recent experience here: https://discuss.moodlebox.net/d/118-performance-deteriorated-a-second-time/. I don't say they have the same cause, but the connection to sophisticated themes is stands out. The good news is that my SD has recovered.
Now to your problem. First thing you should try is to return to to the standard Boost theme. Just add this line to /var/www/moodle/config.php:
You know the commend to restart Raspbian,
sudo reboot? You can aggressively refresh the MB homepage in the browser. Also purge the browser cache.
If all that did not help, you should start deleting the server cache. /var/www/moodledata/muc/config.php is a common culprit. There are much more temporary data there which you can delete, but also essential data, so need to very careful.