High CPU Usage of "SET ROLE"

Hi there,

I'm running PostgreSQL 9.6.10 on Debian and reported some performance
issues with "SET ROLE" a while ago:
https://www.postgresql.org/message-id/CABZYQR%2BKu%2BiLFhqwY89QrrnKG9wKxckmssDG2rYKESojiohRgQ%40mail.gmail.com

Those long running queries, especially "SET ROLE", still persist until
today.

I just recently increased the number of PostgreSQL instances and added an
AWS Aurora Cluster (9.6.10 with db.r4.2xlarge). AWS offers a tool named
"Performance Insights" and shows some really high CPU usage for "SET ROLE"
queries. See attached image or just click here:
https://i.imgur.com/UNkhFLr.png

The setup is the same as reported in the above mentioned post: I use more
than a thousand roles per PostgreSQL instance and set the role for every
connection before executing actual statements. My pg_class consists
of 1,557,824 rows as every role has its own schema with more than 300
tables.

I'm currently building a simple docker test setup with pg_bench to
reproduce the decreased performance when executing "SET ROLE".

I'm aware that AWS Aurora is a proprietary version of PostgreSQL. But I
somehow have the feeling that my experienced abnormalities with "SET ROLE"
in vanilla PostgreSQL occur in AWS Aurora as well. And the high CPU usage
reported by "Performance Insights" may be a hint of a performance issue in
PostgreSQL.

I'm quite lost when reading the PostgreSQL source code (due to my inability
to read it :)) but maybe one of you guys has an idea about that?

Regards,
Ulf