APFS vs Mac OS Extended (Journaled) for external HDD
APFS is known to not perform well on spinning disk drives (non SSDs).

Developer of Carbon Copy Cloner:
An analysis of APFS enumeration performance on rotational hard drives | Carbon Copy Cloner | Bombich Software

OWC/MacSales (who probably know more about Mac hardware than some people who work for Apple:
Using APFS On HDDs ... And Why You Might Not Want To