BUG #15953: performance issues using 11.5-1.pgdg+1 over 11.5-1

The following bug has been logged on the website:

Bug reference: 15953
Logged by: richard
Email address: easteregg(at)verfriemelt(dot)org
PostgreSQL version: 11.5
Operating system: Debian Sid x64
Description:

hi,

i upgraded my server from 11.4-1 ( note, this was the debian package before
) to 11.5-1.pgdg+1 which uses the postgresql apt repository package. after
this upgrade, i noticed a lot more system load and all 4 cores of my server
hit 100% usage by the database.

visualized you can see it here:
CPU usage https://verfriemelt.org/pg-issue-01.png
TPS https://verfriemelt.org/pg-issue-02.png

at ~11:10 i installed the new version, afterwards you see the cpu hit 100%
usage and than followed some desperate measures to get the database to
perform normal again, mostly reindex and vacuum full on the given database.
at 14:30 i wondered and installed the debian provided 11.5-1 package and
everything went back to 'normal' without any other changes. there was not a
single cause like a long running query. just everything went a lot slower
and used more cpu. there were no obvious differences in query planning.

the two versions were the following:

# apt-cache policy postgresql-11
postgresql-11:
Installed: 11.5-1
Candidate: 11.5-1.pgdg+1
Version table:
11.5-1.pgdg+1 500
500 http://apt.postgresql.org/pub/repos/apt sid-pgdg/main amd64
Packages
*** 11.5-1 500
500 http://ftp.de.debian.org/debian sid/main amd64 Packages
100 /var/lib/dpkg/status

using pg_config --configure i found only one difference in the build
options, which is --enable-cassert on the postgresql provided version.

whast the deal?