Doing:

netsh interface tcp set global autotuninglevel=normal

appears to fix the performance for single threaded speed tests.
Now, I wonder why it was set to "disabled", I have no idea.

Thanks, at least I know now these servers are not fundamentally limited for single thread performance. It does look to be an OS config issue.

Probably some software that thinks it knows better than the OS vendor!

At a guess; some software I'm about to find out no longer works properly...

So it looks like yet another reason to hate Hamachi, that one of my clients insists on using!
https://community.logmein.com/t5/LogMeIn-Hamachi-Dis...
GRRRR.

I also heard of the one you're suggesting however did not like it as I think there were a few issues when i was trying to use it.

I find copying large files (my weekly backup) is quite a good way to check I'm getting over 900 Mbps from my gigabit LAN.

I would agree but it's worth remembering that you may be backing up onto a NAS or server of some description with multiple HDD's or SSD's potentially even cached on M.2 SSD's which would get you those speeds but I know I have a Synology 2 Bay NAS and with 2 WD RED 6TB HDD's and I would never get that kind of speed reading or writing onto it. I know my computer can more than handle 5Gbps even 6 over network to writing on the M.2 SSD's.

One suggestion I would make might be to try to check for updates as one windows update, I have had in the past made the computer think 100% usage was whenever it thought the connection maxed out when I could see on the Switch but also testing in Netstress that was only getting 200 when a week later it was back to the actual max after a load of updates.

Although I don't suggest the latest feature update as it seems to be causing a lot of Blue screens and is deleting system files which has caused me to reimage the same PC 3 times in one week so make sure to backup first if you risk windows update, I'm sure we all know what i mean.

We know the hardware is good because a different OS booted from USB worked, so it is a case of working out which part of the Windows system on that machine is slowing down.

I guess there is an answer that any windows user would say to that, once a solution is found a week later there will be another update that breaks it in another way again. Just like the blue screen printer driver issue last month.

I'm not sure if anyone has said this but has he tried to uninstall the network driver and manually override the install by trying to find the driver beforehand or even doing a force driver update as I know that's worked for me before.

All of this windowsness is why I have a Mac for network testing as I know that this is so unlikely to happen against windows where network's don't seem to be supported except as a fun joke to tell linux users.

In reply to a post by RR_The_IT_Guy:

I thought i would suggest the software that a lot of people i know use and its free its called netstress by nuts about nets, here is a link

You download it on two pc's and it maxes out the link if you set it up in the settings, I know it's useful to identify issues as It has helped me to identify bottlenecks in the past so I would strongly suggest trying it.

I couldn't get NetStress working on my main PC. Said something about shared adapter.