That fact he seems to have this problem only on one of his PCs in the household, using the same network, suggests it isn't probably related to that.

My only guesses is that some "security suite" causes those problems on one PC, or it has some oddball network drivers that cause issues (albeit why wouldn't the drivers affect Steam as well...?). I recall discussion how at least on some laptops some network drivers, which were supposed to "boost" connectivity and have all kinds of blah blah extra features, just caused extra problems for most people, and it was better to use some vanilla drivers.
Good luck, and at least at first, don't install any extra virus scanners, firewalls, security suites or special network boosters/drivers, use only the vanilla ones that Windows provides. Try it at least at first with as vanilla Windows installation as possible.

Good thinking doing the reinstall by Media Creation Tool. When I do that, I even delete all the old recovery partitions etc. from the hard drive, and let the new pristine Windows installer recreate them from scratch. Then again, if you have some specific utilities on that original recovery partition. then you have to reinstall them manually, in case they really are that useful. Like on my Dell laptop, I manually installed the Dell Command Center utility as it checks for any BIOS/UEFI updates or specific driver updates for both the laptop and its docking station.

I definitely think it must have been something in my previous installation that was causing issues. I didn't have any extra security software, but maybe some remnants of some motherboard utilities. I didn't have any installed at the time but I know they can be sticky.

On a fresh install, with full Windows updates and just the bare essential drivers I am now getting a consistent 80-100MB/s downloading through GOG Galaxy now. Much better!

The best strategy at this point is to download the offline installers and then install the game into your c:/gog games folder. then let gog galaxy auto detect the game once it's launched from your end.

You're better off installing games to drive D or further.
Only time where a drive C would be valid, would be on a single drive laptop.
Something's still not quite right with your network.
Earlier in the thread, you'd said you were getting 700MB/s on your laptop.
So what's still slowing down your speed on your desktop, when they should be the same speed.

I've not bothered to read all the replies here, so I am wondering if you are aware that Galaxy can also download the Offline Installer files. That means you don't need to use browser links etc. Just investigate its options to find out how.

I don't use Galaxy, but have a few times in the past, and many folk here use it to download the Offline Installers.

In any case, there are currently server issues with GOG, especially if you live in certain parts of the world, including parts of Australia. I certainly get really lousy bad download speeds at the moment from GOG.

The 700 from earlier was in Mbits/s which is around 90MB/s. That was the speed from the Windows Media Creation tool which was reported differently, so the speeds are both in the same ballpark.