Which backend did you use for sync?

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.