Encrypted Time Machine backup is super slow using an encrypted USB volume?

Based on the comments by user Tetsujin and this apple community entry the "answers" are:

• Yes, a full disk encrypted TM backup to a USB3 drive is slow. Overall performance varies greatly, many small files need much more time compared to fewer large files. Overall, my 126 GB backup took about 12 hours.
• The one thing that you can (and should) take care: format the whole disk as "Mac OS Extended (Journaled, Encrypted)". If you don't select Encrypted, TM will later encrypt the drive on its own, which requires (again!) plenty of time (besides the time it took to just write the backup itself). My initial approach of using APFS: don't do that!

Note: the one other thing I learned: you can easily suspend that first TM backup, simply by clicking the "skip this backup" menu item while it is running.