Yousuf Khan wrote:
[...]

VSS is used on all of the backup jobs. None of the others exhibit this
behaviour. In fact, I've experienced this issue for nearly a decade now.
The problem started on Windows XP, continued on into Windows 7, and
continues to plague me in Windows 10. This particular folder has also
been migrated around from HDD to SSD, to a 2nd SSD, etc. So it's not a
problem that is specific to HDD's or SSD's, or to any particular version
of Windows.

I'll tell you what this folder is. It's actually my Thunderbird News
folder (exactly what I'm using to ask this question here), which exists
under the my User folder structure. The problem was discovered when I
started doing daily backups of my User folder and discovered that the
User folder was taking forever. After investigating it some, I figured
out that the problem was this particular substructure under News. Once I
excluded the News folder, backups finished 6 times faster! So I moved
the backups of the News folder to their own job, and let the rest of the
User folder get backed up separately. Before, you ask, I only backup the
News folder once a week, but it's still a pain in the ass watching it
take so long even once a week.

If there are 580,000 files in the News folder, then you've probably
configured your Thunderbird News account(s) to use one file for each
article instead of one file for each newsgroup.

If so, it's probably best to bite the bullet and convert to one file
per newsgroup. That probably needs an export and (re-)import and
probably will be time-consuming, but at least then you'll solve the
actual problem.

FYI, my setup - not Thunderbird - has nearly a million articles, but
only some 600 files.

Some other background. When this particular backup is happening, it's
not the drives that are showing as busy, it's the CPU cores! 4 out of
the 8 cores on my FX-8300 are fluctuating between 50% to 100% busy,
whole the other 4 are not that busy.

My guess it that this processing is spent getting the hundreds of
thousands of files into and out of the file system cache.

Yousuf Khan