The latest heavy latency can not only be tracked down to #4, but got resolved by disabling the redis cache. Next to filesystem optimizations setting on the host kernel seems to be necessary for operating redis properly. In the logs we find:
redis_1 | 1:M 05 Dec 2018 08:37:31.403 # WARNING you have Transparent Huge Pages (THP) support enabled in your kernel. This will create latencyand memory usage issues with Redis. To fix this issue run the command 'echo never > /sys/kernel/mm/transparent_hugepage/enabled' as root, and add it to your /etc/rc.local in order to retain the setting after a reboot. Redis must be restarted after THP is disabled.
Sources from @gandhiano 's research from the matrix: