Basically trying to work out why Windows 10, Update is takes so long an I have ruled out:

Network speed

Lack of processing power

Inadequate memory

Disk speed

My hypothesis is that something is blocking IO, which is slowing down a relatively trivial, but large update. I'm beginning to think Windows Defender is locking the IO threads for the MoUSO process (the newfangled-ish updating infrastructure for Windows). Which is, maybe, artificially (deliberately?) massively slowing down the update*.

Assuming the update can be regarded as trusted it seems redundant and careless that Windows Defender is using at least one core at close to full utilisation. On a fresh install.

I am fairly ignorant about Windows - about the same as the average person reviewing it.

* Or there is no IO process/thread blocking it's just - see I don't understand how it can be that slow. I get why Android/iOS updates are slow but not Windows 10 onwards with enough resources. Unless it's doing something really odd and computationally intensive. Except it isn't - I can monitor it quite well inside a KVM container.