Regarding: https://forum.zenphoto.org/discussion/comment/1981178/#Comment_1981178 and html_static_cache
No, I don't see this one in any plugins-overview of my installs.

Where should it be? Where should it be de/activated?
Are there any settings possible?

Well it is actually named static_html_cache ;-) But it is an included plugin in the release package for years. It is off by default.

Now we're talking miley: )
Thanks for clearing things up.

Enjoy!