UI login and logout slowed by many group members

Login and logout to the COmanage UI became increasingly slow, taking over 60 seconds for a single account. The rest of the interface responded quickly, loading most pages in about a second.

Increasing memory_limit to 1024MB and max_execution_time to 60s in php.ini was necessary to prevent timeouts and errors.
Increasing RAM from 2 GB to 3 GB didn't help.
Upgrading to PHP 7 helped somewhat; memory_limit was reduced to 512MB, and login/logout time was decreased about 30 seconds. But top and vmstat 2 commands showed large CPU and memory spikes during login/logout.

Deleting all cm_co_group_members records with actor_identifier = CSUAPIUser (created by our API calls) resolved the problem. Login now takes about a second, and the rest of the interface remains fast and functional.

We currently don't need the cm_co_group_members records because we use Grouper for group management, but I was just testing a variety of COmanage features.
There were about 4 cm_co_group_members rows for each co_person_id, with 4 different co_group_id values:

• csumembers - Colorado State University Members - created by my API calls
• CO:members:active - automatically created
• CO:members:all - automatically created
• id=0 - created all at once, possibly by my API calls by mistake

So the issue may have been due to bad data I created, and may not be an issue for most COmanage installations, but I am reporting it here in case it's a more general performance issue with group members.