CPU - Current Gen4PCIe GPU on x4 -

For instance, a x16 slot with only 4 PCIe lanes is quite common Computer bus interfaces provided through the connector are PCI Express

You're referring to NVME based M2 SSDs, correct? But yes if the SSD is PCIE spec then of course the speed will be less when used on PCIE

m2 nvme slot 2x slots M2_1 slot Supports PCIe Da kann Slot 1 PCIe und und

m2 expansion slot Biostar ; 1, 1 x AMD Socket AM4 CPU ; 2, 4 x DIMM DDR4-4933+ ; 3, 4 x SATA III Connectors ; 4, 1 x PCIe x4 Slots ; 5, 1 x PCIe x4 Slots