What is PCI Express? Do PCIe lanes, slots, and versions matter?
If your build has a large graphics card, it may hang over one of the smaller PCIe slots, in which case you would need to skip a slot and use the bottom PCIe slot (as is the case for our example build).
Graphics Cards - All series｜ASUS Global
So even though your Graphics Card fits nicely into a mechanical x16 PCIe-Slot, this doesn’t mean that Slot will run at x16 PCIe-Lanes.
PCIe Slots & Lanes Explained: Choosing the Right Connection (Fast ...
A PCIe x16 slot is typically used for connecting a high-end graphics card to a computer. Graphics cards require a large amount of bandwidth to transfer data ...