Gen2 x4 or gen3 x2 lanes would be a good explanation, it really depends on the motherboard. I had only 6 Gbps with my MCX311A-XCAT and it turned out that my mobo supports only gen2 x2 on the second PCIE x16 slot even though the first slot is gen3. I had to change motherboard to be able to fully use SFP+. I am thinking on QSFP+ now. If we can buy these cheap cards and convert them to real Mellanox, then SFP+ and QSFP+ have almost the same price.Just a guess, you may have one card in a system's pci-e slot that's only gen 2 x 4 lanes which would net you a max of 16 Gb/s even though the 40Gb link is active.