I have been having issues with international speeds recently and at a loss as to why.

@NickMack and the team at 2Degrees have been an awesome help with this issue; sending out some test gear to assist with troubleshooting the issue.  (Thanks again guys!)

The conclusion of the testing with 2D was that my network, Internet connection and PCs were all fine, the problem was something specific to Windows.  Speeds in Linux were fine.

However, nothing I tried made any difference until I discovered this article:
https://argonsys.com/microsoft-cloud/library/windows-network-performance-suffering-from-bad-buffering

As a result I discovered that iPerf3 v3.1.3 for Windows (the latest stable release) comes bundled with a version of Cygwin1.dll (2.5.1) which has a hard-coded TCP window size setting of 278,775.

This causes results to be limited to 12.6Mb/s per thread.

This is why iPerf speeds in Linux are much better that in Windows.

Updating the Cygwin dll to the latest version resolves this issue and I now get the International speeds I am expecting when testing with iPerf.

I have informed the iPerf devs of this issue.

As a side-note, speeds with Speedtest.net and nperf to the US are still pretty much rubbish.  I'm not sure why, because they used to be fine.  At then end of the day iPerf (with updated Cygwin1.dll) is the most reliable testing method by far.