How can I speed up Time Machine backups that intermittently taking way too long?

Question

I've long been amazed how fast time machine backups can be these days (well, years); even for really gigs of data. But recently sometimes my backups take forever. It seems to be independent of how much data needs to be transferred, sometimes it's just a hundred meg or so. This is from a macbook air, onto three different USB drives (in case one fails or our house burns down). So it's not about any particular hardware, nor wifi.

I found one possibly applicable solution that suggested running file utility, but that hasn't seemed to help. Stuff about disk throttling I wouldn't think applies since it was never a problem before.

Another symptom: when this is happening, I sometimes need to leave, so I try to abort the backup, and this also takes a lot longer than I would expect from (long) experience.

Answer 1

While waiting for an answer, I ran the disk doctor again, and I noticed that it was taking a very long time with the name of a time machine file from a few weeks ago. Of course, this file didn't prove to be searchable from the finder, so while poking around trying to remember how to find things from unix (which is slow given how huge disks are these days), then trying to figure out a likely subdirectory to search, I came across this great answer https://apple.stackexchange.com/a/309950/85075 This not only explains how to find the files, but that ones that say dataless are "all kinds of trouble", and what unix incantations you need to delete them. As you might guess, the file with this name was indeed "dataless".