My point is that while the old HFS+ based Time Machine was performing ok on spinning disks, the new APFS based TM backups are excruciatingly slow and take ages to finish. I did actually compare both types on the same hard drive and even then the difference was very obvious.

It's definitely APFS. I've had the pleasure* of HFS+ and APFS on the same external HDD and APFS is horrible. Given it is optimised for solid-state drives[0], it's understandable there'd be some slack but it is absolute horse paste.

(If I could create an encrypted external HFS+ volume, I would but macOS hasn't allowed that for a while as far as my experience goes - it's non-encrypted HFS+ or encrypted APFS.)

[0] https://support.apple.com/en-gb/guide/security/seca6147599e/...