Sorry to bring up an old thread, but this same thing happened to me and not sure the solution mentioned would work for m.e
I have two Darktable installations accessing the same picture share (one Windows, one Linux). The pictures I'm accessing (~43,000) are on an ssd native to the Linux system. I have it Samba shared to the Windows box. Network connectivity between the Windows and Linux system is gigabit (they sit a few feet from each other and on the same 5 port switch).
I have "check for updated xmp files" checked on both Linux and Windows installation because what I edit on one side I want to show up on the other side. Historically, I've used Digikam, but I no longer am using it.
Darktable on Linux opens up almost immediately (even with the "check for updated xmp files" checked). But on Windows, it's chugging through the xmp files looking for updated files. This can take upwards of hours.
I started using Darktable on Windows this past week. I first imported the pictures/xmp file metadata into Darktable. I then restarted Darktable, only to find that a process in the background was running at about 5-10% CPU and some network activity (and no UI showing up). When the UI finally showed, it said that all my xmp files were updated (they were about 3 minutes newer than the expected time - which I found odd). So I told Darktable to update the database with the "updated" xmp files. It chugged through those.
I restarted Darktable again, and it once again spent an hour or so before showing a UI. This time it didn't mention that there were updated xmp files.
After reading the above, I turned off "check for updated xmp files", restarted Darktable, it started in a few seconds. Great, but maybe not great.
I don't know if that's a solution. If I modify a picture on Linux, the Windows side won't see the change, and more importantly, possibly overwrite those changes if I edit the same picture on Windows.
Are there better ways to share pictures across multiple installations of Darktable?