I have an external USB3 disk connected to my Mac Book Pro. I first formatted it with GUID/APFS and encryption, and used the "atto disk benchmark" app to measure performance. It showed about 100MB for read/write.

Then I turned to Time Machine, and selected that volume as encrypted backup drive.

But TM writing the backup is SUPER SLOW. It takes about 100 seconds for 1 mega byte to be written. Thing is running for 20, 30 minutes by now, and I see 17 MB out of 112 GB of progress. It is so slow that TM won't even give me a "time remaining" information.

So: is there anything I am missing? Or should I just forget about using such kind of drive for an encrypted backup? (doing an encrypted backup is a hard requirement)

Edit: turns out that TM converts the file system to Apple_HFS under the covers. And: speed is actually fluctuating, probably depending on file size (now at 4 GB of 112, after 90 minutes or so)