Later you can read you should use specific slots on the motherboard for 10GbE PCIe NICS (and probably
other high throughput cards):
Going back to the earlier discussion about PCIe root ports being integrated into the E5 CPUs, this leads to a consideration for the placement of PCIe cards. Make sure your high-speed cards aren’t inserted in a slot that runs through the C600 chipset southbridge. Make sure that you are using Gen2 x8 slot, and make sure that the slot is actually wired to support a x8 card (some slots on some systems have a x8 connector but are only wired for x4 throughput). Johnson recommends using either LoM, slot 2, slot 3, or slot 5 for 10 GbE PCIe NICs; this will ensure direct connections to one of the CPUs and not to the southbridge chipset.