In Nextcloud Files > Settings, it gives a path like this: https://cloud.example.com/remote.php/dav/files/user.name/

I'd found the fastest alternative to be using Mountain Duck, but that does come at a cost.

But then, inspired by some reading on macFUSE, which I have installed, but I don't think it's a required part of this solution, I just tried changing the beginning of the URL to sftp, so: sftp://cloud.example.com/remote.php/dav/files/user.name/

Hey presto! Super fast Nextcloud filer server mounting, without having to use the Nextcloud app for copying and synching some or all folders locally, and in duplicate.

Search engines seemed to yield very little on my searches for "mac nextcloud webdav slow" several times over the last year or so, but a little guessing paid off. Hope it might also help some others that have this issue.

In related news, the path for mounting drives in Linux desktops using that file browser, is through Other Locations > Connect to Server, and changing the beginning to davs, so davs://cloud.example.com/remote.php/dav/files/user.name/

And with Windows, just the https://cloud.example.com/remote.php/dav/files/user.name/ value you find works just fine.

More on the subject of Windows mounting of Nextcloud here: https://forum.cloudron.io/topic/6514/windows-can-connect-to-nextcloud-webdav-but-then-loses-the-connection-and-can-t-reconnect

I'm thinking that it would save Nextcloud users a LOT of frustration and searching just to have all three of these values showing and labelled relatively in that Settings panel.

EDIT: Doh! Just realised this only works on macOS because I also have Mountain Duck installed, and that is picking up the sftp URL from the finder and then brokering the connection. Dammit, looks like that neat little $39 app is the cost of doing this on a Mac - but if you have a Mac, you already know that you pay a premium for speed beauty.