My Time Machine HDDs are definitely less than 5 Gbps. Apparently much less. I have downloaded the AJA app, and it won't work on Time Machine drives. Looks like nothing but Time Machine itself can do anything to them. If a TM drive is failing, you can't copy off the backups! Well, hopefully it's at least robust, ransomware-proof, and will restore files and folders correctly, unlike my last restore operation with an older version of TM, which I had to do due to a failed drive. It put the Movies and iTunes or Music folders in the wrong places, and missed some files, IIRC! Probably best to have two running in parallel, which is what I do. For really old backups, I guess the only way to be sure you can go as far back as you like is to buy that with an offsite/online backup service.
Anyway, the claimed speed on these TM drives (OWC), at least currently, is "up to 252MB/s real-world speed." But that's probably for a large file, not something like snapshots, which have many thousands of files and folders. I assume there's a lot of overhead with that structure!
The great thing about Time Machine is that it makes it easy to restore files that go into a database, like Mail, TV, Music and such. macOS does it for you!
Running AJA on my external SSD I get about 800 MB/s average. Not bad! The M1 Mac gets like 2300 MB/s!
I still think it takes an incredibly long time for "First Aid" to run if you have a fair number of snapshots. Wow, I just ran AJA on the internal drive and got 3139 MB/s write and 1898 MB/s read!
The external SSD: Just got 838 MB/s write and 766 MB/s read. I think the advertised value was "up to 1025 MB/s" or similar.
I've had mixed success with WD. One drive was broken right out of the box! I couldn't even initialize it. I could hear the head assembly going back and forth in spurts maybe 1/2 second apart.
I had a SanDisk 2TB SSD that would occasionally brick on me, and now it's just dead. I've heard that a lot of them were going south!