are you sure that is because of apfs? hdds are usually much slower, no?

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/...