PCIe : Everything you need to know, from specs to compatibility

x16, 1 PCIe x16, 1 PCIe x1, 1 Key E for Besides the onboard LAN port, users may also choose wireless connections by slot

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_1 slot PCIe PCIe x 16 Slots 7 · 6 · 5 · 4 · 3 · 2 · 1 SSD Slots 5 x Ultra-Fast Storage：4*PCIe x4 slots; Efficient Overall Thermal：VRM Thermal

m2 slot mainboard The lower slot on the Asrock board is using one of the SATA lanes So you won't get any speed advantage PCIe pastedImage_2