I am using Joplin on Linux, Android and Windows but I am finding that Linux sync takes a lon time, like minutes to update a single note update. The android and Win versions have no such issue. ALl of these are on the same wifi network, and the Nextcloud server is on the same network.
There have been multiple threads about this issue. The sync process is not actually slow itself just the display of the sync status gets stuck. Hiding and unhiding the side bar (e.g. by pressing F10 twice) updates the display or at least advances it by a step.
It’s annoying but not really a major issue.