Those results were worrying me, so I reran them and used a few more data points. Those results were much closer, again the 6 coming out ahead but with the 6s coming in a only a second slower. I guess the 6s was busy doing something in the background on my first attempt

I decided to do some tests on a Windows machine too, so I used Explorer to do some transfers on a suitably fast PC. Those results came out more how I expected, with the 6s being 24.1MB/s and the 6 being 18.4MB/s.

All these speeds are well within the practical throughput of USB2 though. I've just done some transfers from a USB2 connected hard disk to the Mac. Those transfers are at ~40MB/s.