Motherboard Ports: What They Are and How They Work - Lifewire
There are two types of PCI slots: PCI and PCI Express (PCIe). The PCIe slot is a larger, faster version of the PCI slot and usually houses your graphics card.
Which PCIE slot in one GPU configuration - ROG Forum
If your motherboard has more than one PCI Express slot, it may not be immediately clear which one is going to be the best for your graphics card to use.
GIGABYTE H610M H DDR4 Mainboard
* For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16 slot. 1 x PCI ...