Considering I've seen video's of the highpoint 7101-a pulling 12,000 MB/s in a 1x4 1TB SSD setup and 24,000 MB/s in a 2x4 1TB SSD setup in a hackintosh with true x16 PCIe 3.0 slots... More speed is expected, but perhaps not in a 7,1 Mac Pro. Which is a shame considering the premium paid for the platform.
Clearly, it's not your SSD's or the highpoint 7101-a, and likely a limitation of the 7,1. architecture. Check out this video where the Sonnet M.2 4x4, equipped with the same Broadcom PEX8747 PCIe switch, barely hitting 7,000 MB/s on a 7,1.
Just to clarify, the video title calling the speed insane should say insanely slow. Clearly something is amiss with the 7,1's PCIe bandwidth. Is the slot you're using setup as x16?