I have an external USB3 disk connected to my MacBook 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 Time Machine writing the backup is super slow. It takes about 100 seconds for 1 megabyte 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 Time Machine won't even give me a "time remaining" information.

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.)