I was thinking maybe mariadb needed to be tuned differently from 
RHEL8->9 but if I log into the mariadb container and query the 
database it seems equally fast; I can do 100 simple queries in a second.
 Where should I be looking for the source of this delay?

1) It seems you are "only" testing with keystone (users) or, better put, you already observe the issue at the keystone API? Are any of the other APIs slow as well?

2)  Did you init a new database (fresh schema) or did a "db sync" to the existing DB and applied all the schema updates? 

3) Did you check the upgrade logs? Are you certain you applied all the upgrade steps including online updates? See https://docs.openstack.org/keystone/latest/admin/upgrading.html

4) Can you maybe init a new DB and compare the table schemas to see if there is any index or similar missing on your upgraded database?

5) Turn on slow query logging, preferrably using the extended capabilities https://mariadb.com/kb/en/slow-query-log-extended-statistics/ to find any query that is slow or not using an index.

Regards

Christian