Some random things I can think of:
-What % full is it?
-What pcie mode does magician indicate it's running?
-Is it in the correct motherboard slot? Not experienced directly with that board
-Are you on the latest firmware?
-latest Samsung driver or windows driver? Samsung driver I want to say is 3.1 but don't quote me on that
-Is your intel MEI driver up to date?
Is it a brand new 970 EVO or has it been running for a while with a large TBW? My 850 had the same problem and i did a bios secure erase. Brought the speeds right back up.