USB 3.1 performance
Since USB 3.1 is not yet part of the chipset, the manufacturers have to use extra controller chips to support the new interface. We test the USB 3.1 connections with an external SSD enclosure from ASUS. Inside of the enclosure are two Samsung 840 EVO SSDs with a capacity of 500GB each, in a RAID-0 configuration. It should be clear: the bottleneck absolutely doesn't lie in the storage device.
ASUS offers an USB 3.1 ''Boost'' driver with its Z170 motherboards, which clearly gets its job done. We measured speeds above 750 MB/s with the ASUS board. The motherboard of MSI didn't score as well, but this is partly because we enabled C-states. This limits the idle power consumption, but also causes worse USB 3.1 performance, especially with MSI.
The ASUS Maximus VIII ranger performs the best in this test. The ASRock does less well, and the MSI Z170A Gaming M5 finishes last. The USB 3.1 performance of this motherboard is sadly one of the worst we've ever tested. This isn't solely because we've enabled C-states, since many cheaper boards from MSI perform better in this test.
We couldn't get the Gigabyte to work, because the Intel Alpine Ridge chip seems to not be compatible with out test configuration.