Is APFS on spinning media really this slow?

I'm helping out a former colleague with her 2019 iMac. She was complaining that the computer was running really slow. I connected remotely, and sure enough it was dog slow. It is running Mojave because she doesn't want to lose access to a few 32 bit apps that she still uses.

She didn't know about the differences between SSD and HD when it was purchased, so it has the base 5400RPM 1TB drive in it. She didn't purchase AppleCare with it.

Launching Word takes 30 seconds. Launching InDesign takes 30 seconds to the splash screen, and another 30 to be able to open a menu. I cloned her drive to an external 2.5" SSD I had in a UASP capable case. Performance jumped to where it should be. Word and ID open in under 3 seconds.

OK, so maybe the HD is bad. I've seen them fail in ways that slow performance to a crawl... except... Blackmagic Disk Test says the drive is happily doing 90+MB/sec. read and write. Right where it should be for that drive.

Is APFS overhead for spinning media REALLY that bad? Or is there something else going on here? I cloned the SSD back to the HD after erasing it, and got the same slow results.

I know the best solution would be to install an official Apple PCIe blade inside the machine, but I don't feel comfortable opening the "glued shut" Macs, especially when they are only a year old. She really doesn't want to deal with the Apple store right now. Is there anything wrong with sending her home booting from the external SSD?

On a side note, I was really hoping there was an external thunderbolt 3 case that supported the Apple PCIe blades. Then when she did feel comfortable going to the Apple store, it would be just a matter of moving the blade from the chassis to internal slot.