The speed is going to vary based upon the fact if there is a card installed in slot 5 or not. If there isn’t; Slot 6 should be able to run a PCI Express* Gen 3 x8.
thank you for your answer.
S1200V3RPO has only one PCIe slot, which is slot #6.
Indeed if I connect a PCIe GEN3x16 device it's trained only at GEN2x8.
I am in contact with INTEL Customer support for this.
I'll let you know their answer.