NVMe SSD to PCIe x4 Expansion 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

With PCIe Gen 5 SSDs becoming more available, we're going to look at the differences of Gen 4 vs Gen 5

m24 slot Like USB, PCIe is forward compatible This means that a device which requires PCIe will work in a PCIe or PCIe slot Slot Size Lane Width

m2 pcie 40 in 30 slot USB Gen 2 x 2, USB Gen 1 x 4, USB x 3 (Optional Mini PCIe Slot x 1 Micro SIM Card Slot x 1