From: http://www.overclock.net/t/1579548/asus-z10pe-d16-ws-owners-thread/30

Here is a comparison between mounting the Samsung 950 pro into the on-board M.2 slot (on the 
left in the picture) versus using an nvme to Pci-e adapter. I am using an inexpensive adapter 
that I got from Amazon for only C$19 (sold by JZLL)! As you see there is a significant improvement 
in the seq reads but not as much in the seq writes while random 4k read/writes show almost no improvement. 
Since this nvme drive has only the operating system on it but no personal data, it seems I get no practical improvement by installing my system drive into a pcie slot over the on-board m.2 slot.