Original message form customer:

(…) Since we activated the slow query log
a few days ago, we saw 4 recurring querries in the logs. Two of them
were on the tokens table. The first one was fetching the session tokens
and the second one checking expired tokens. Both toke 20-30 seconds.

We created this morning two additional indices on this table. The first
on the expires column, using btree, and the second one on the type
column, using hash.