"Beide bereiten mit anderen Protokollen wie NFS, SMB, usw. keine Probleme."
NFS and SMB are network file systems. WebDAV is not a network file system. You might read https://www.rfc-editor.org/rfc/rfc2291.html about what WebDAV was intended for. The intention was that editing tools have built-in WebDAV-support (like e.g. LibreOffice or MS-Office). That there are now WebDAV file systems for almost every operating system does not change this. WebDAV lacks many features that are required for an efficient file system, e.g. real directories or writing part of a file. The main purpose of davfs2 is to allow programs without built-in WebDAV support to edit WebDAV-resources. The only reason why it is a file system is that all editing software can interact with a file system.

"WebDAV läuft über Dolphin direkt, über Kodi, Cyberduck und Windows also besser als über davfs2."
Otherwise would be strange. When Dolphin et al. use their WebDAV-module to access WebDAV-resources they know that it is WebDAV and hopefully behave accordingly. When these programs access a davfs2 file system they treat it like a local file system and do a lot of file system actions that are cheap locally but may be very expensive with WebDAV. You really should use the debugging features of davfs2 to watch all that file system actions of that tools.

"Wenn ich das jetzt richtig gelesen habe, soll es angeblich am caching liegen?"
Why do you think that? Caching makes most operations much faster.

"Aus meiner Sicht ist Owncloud bzw. WebDAV per HTTP am fortschrittlichsten, bietet alles und reicht allemal."
Except WebDAV not beeing a protocol for a file system.

There is one thing that davfs2 could do: it could download only part of the file. That would be useful for tools that open every single file just to create fancy icons. Not opening every file but only those you want to edit would be better.

Werner