Hi,
I did some checking and those Marvel controllers do run a little slower, isn't there a Intel port available as you have 6 of those?
Check this post I found via Tomshardware: Link Removed
I also checked your driver support and your board is only supported up to Windows 8.1. so might be worth going back to an earlier os although I still doubt whether you'll get faster speeds unless you use rapid mode via the Samsung Magician app?