The FIRST M2 port is connected directly to the CPU PCIE LANES (x4 lanes - PCIE 3.0) and capable of supporting NVME's with a max bandwidth up to 4 GB/sec.

The SECOND m2 port is connected to the CHIPSET (the chipset subsequently has a 4X PCIE 3.0 connection to the CPU), moreover - it's only a x2 LANES so max bandwidth is limited to 2 GB/sec.

Stick to the first M2 port..