You usually get better sync performance by either self-hosting or using Joplin Cloud. I don't know of any good WebDAV provider, let alone a free one, and even the paid ones have limitations that make them unusable for sync.
OneDrive and Dropbox have a free tier and work with Joplin, but they throttle the connection. But I guess it depends on how many notes you have and how you use Joplin - a lot of people use OneDrive or Dropbox and it works good enough for them.
Fastmail comes with WebDAV. I used it with Joplin, but it pushes a very large number of files to the server. I have a paid OneDrive account. Found out that sync didn't work with OneDrive (random files just won't sync). Maybe they've fixed it, but I was not impressed.
Nextcloud has a surprisingly inefficient WebDAV implementation. It's not clear how they got it so wrong compared to, say, Nginx or Apache WebDAV, but they did. Maybe they are doing some processing on each request, creating thumbnails, checking for shared files, locks, or something that's not really necessary but it is slow as a result. Perhaps some config changes could indeed help though.
You usually get better sync performance by either self-hosting or using Joplin Cloud. I don't know of any good WebDAV provider, let alone a free one, and even the paid ones have limitations that make them unusable for sync.
OneDrive and Dropbox have a free tier and work with Joplin, but they throttle the connection. But I guess it depends on how many notes you have and how you use Joplin - a lot of people use OneDrive or Dropbox and it works good enough for them.