Should you buy a PCIe or SSD?

pcie 3 slot   Dan pcie 50 x16 slot

In this video, I tested the PCIe gen 3 slots to see if there is any performance difference when

First of all, I know that a PCIe 4 SSD NVMe will be running at gen 3 speeds and bottlenecked by the PCIe 3 slot But I'm wondering if there are PCIe slot will run at the lowest denomination speed The most common PCIe slot currently in use are PCIe x16 slots, PCIe x4

register free rm3 e- wallet slot An x4 PCIe device has a theoretical max transfer rate of MBsec according to wiki  PCIe bifurcation PCI Express bifurcated Riser Card ITX to ATX x4x4x4x4