How can I test the speed of my external Time Machine drive?

[Killroy]

That might help with file corruption. Won't help when TM writes a pointer with the wrong values to a file. The classic resource for understanding the Unix file system (before APFS) is:

The Design of the UNIX Operating System

The Design of the UNIX Operating System

www.amazon.com

Here's a brief discussion about Time Machine internal structures:

Time Machine to APFS: Backup structure and access

Unlike HFS+ backups, those on APFS volumes look different depending on how you try to access them. What you see in the Finder is an illusion not available to other apps.

eclecticlight.co

"Disk Drill" Is whole disk recovery not just files. You recover what you can to a good disk.
Also APFS is a cut down ZFS. Apple all most went to it but internal politics got in the way. There's different rumors about why ZFS was not the main file system for Apple used.

 

[HDFan]

Nothing in post 26 is relevant to a discussion of a TM file with a bad internal pointer. I have given you some good resources. Your choice to use them or not.

 

[Killroy]

Nothing in post 26 is relevant to a discussion of a TM file with a bad internal pointer. I have given you some good resources. Your choice to use them or not.

You can say the same about post 25.