Which one between APFS and Mac OS Extended (Journaled) performs better as partition store type for Time Machine backup in an external HDD (Magnetic Storage over USB connectivity)?
Note this answer now depends on the version of OSX/macOS you have.
This answer is for macOS Catalina and earlier versions - Big Sur now can use APFS - see Apple's Types of disks you can use with Time Machine on Mac You can also use HFS+ as before.
You can't use APFS as a Time Machine target:
Time Machine drives must be HFS+. That will likely change in the future but currently it is HFS+ only.
Now it can be done:
macOS 11.0 Big Sur: The Ars Technica review …in Big Sur, Time Machine does make the leap from using HFS+ to using APFS… https://arstechnica.com/gadgets/2020/11/macos-11-0-big-sur-the-ars-technica-review/9/