Thanks for sharing back exactly what you did - others might find that handy!
Got CLI access to server? If so there are a few CLI only scripts in code/admin/cli/ related to DB. As we know, DB is the 'heart' of Moodle ... if it's optimized, etc., things run well.
mysql_collation.php, mysql_compressed_rows.php, mysql_engine.php and check_database_schema.php.
Each script has a help screen that shows options to the scripts.
I run them from time to time to assure the DB and tables and columns in those tables are what they should be (I have a bad habit of running things as root user rather than as web service user).
I'd also highly recommend the installation of MySQLTuner - a command line only Pearl script which makes intelligent suggestions on tweaks for DB server - especially important if running a Moodle on a server that's an all in one ... web + DB.
'SoS', Ken