We had the same problem and also posted in this thread Users & Roles slow to load, OJS 3.3.0.6, which has since been locked. Recently we came across this DB bug https://mariadb.com/kb/en/optimization-and-tuning-migrated-from-10122-to-1067-and-queries-get-stuck-u/ and can confirm that the workaround via ANALYZE actually solves the problem for us (afterwards load times less than one second as opposed to 90 seconds) - maybe try this?