Hello, it’s me again 
just wanted to add my latest update on this topic (partly posted also here Nextcloud really slow after installation - #6 by besendorf):
Meanwhile, i made a “bare-metal” test installation on the same host, where my docker container are running. And the bare-metal nextcloud-instance feels lightning fast - sometimes the load times are below 1s. I’m already thinking about moving back to bare-metal or considering the use of VM’s like proxmox. But somehow i still don’t want to give up docker, because i really like the concept.