All of the M.2 stuff I've read in this thread would seem to be correct, but I wanted to add something.

Most modern motherboards with an M.2 slot will be allowing NVMe speeds (up to x4 PCIe lanes through the chipset on modern, standard desktop systems where the CPU's x16 PCIe lanes are tied up with the video card installed), but not all M.2 slots on motherboards are NVMe capable. Some are only SATA speeds, and will likely disable a cable SATA port on the motherboard if that SATA M.2 slot is populated (say SATA 2, the 3rd port, for example, so be aware). But, you'll see this more where there are motherboards with 2 M.2 slots. ASRock AB35OM Pro4 comes to mind.