Check one long running session which reported by dev.
Check session 1382/7489
INDEX RANGE SCAN, it should be pretty fast.
Trace the session to see what’s doing now.
The session is scanning USERS table.
Check FK constraint.
Also, after checking, there is no index in the fk key column in child table.
So updating parent table causes full table scan in child table due to fk.
After creating index, the update completes very fast.