I did a quick test with my Nexus 5, connected to the Windows PC reads over USB2 were just a bit slower than the iPhone 6s.

I did one further test, I used iTunes to transfer a 280MB file from the iPhones to the Mac. Both of them were hitting ~35MB/s.

So I'm guessing whatever protocol phones use over USB to pretend to be harddrives is putting an extra overhead that you don't get with real hard disks connected via USB ?

The fact that both phones are maxing out at the same speed would tend to suggest that I'm wrong and that USB2 is the limiting factor.