WTF is up with Windows Networking?!

Today, in my role as an IT consultant, I was faced with a strange issue at a customer site. The issue manifested as poor internet performance, unresponsive web sites, and MS Outlook failing to connect properly. The fault was eventually traced to a fibre module in their core switch which was causing massive CRC errors on the link.

In order to troubleshoot and finally verify internet performance, we were using a well known online speed test tool to check performance. During the incident, download speeds varied from 50Mbps to 200Mbps on a 1Gbps circuit across all devices, not great. After the fix was applied, we had stable download speeds of ~500-600Mbps on most equipment.

I started getting suspicious that something was amiss when one PC was achieving ~900Mbps download, whilst my customers Surface Book with a docking station was only achieving ~250Mbps, and my Surface Pro 7 was only achieving ~80Mbps with the USB network adapter.

I focussed on my Surface Pro to dig a little deeper, and found that when not connected to the mains power, the Surface running Windows 11 would only achieve 50Mbps over WiFi5, and 80Mbps over Ethernet. When the mains is plugged in, this went to ~180Mbps over WiFi5, and ~500Mbps over Ethernet.

Whilst the power profiles seem to be in play here and adjusting performance based on whether it has mains power I thought I'd try something out. I unplgged the mains and manually set the performance profile to "Best Performance", expecting this to return the performance levels to those seen with the mains connected, however it didn't, we went back to the same levels as before (50Mbps over WiFi5, and 80Mbps over Ethernet).

The final tool in my kit was to boot my Surface using a Linux live USB. I booted up, and connected to WiFi and had download speeds of ~200Mbps, which was an improvement. The biggest shock was when I plugged in the USB Ethernet connection and then had consistent ~900Mbps downloads. The same cable, the same switch, the same hardware.

So, WTF is up with Windows Networking?! Why is there such a discrepency between Windows and Linux? Why does the network speed reduce so much when the power isn't connected?

Clearly there are no answers here, this is more of a rant. I'll keep digging to see if I can find anything else. I fully suspect that not even Microsoft could answer this to a suitable level.