Your spare PCIe slot may not actually be free, and checking your manual matters

Many people assume they can simply install a PCIe or M.2 expansion card without any issues, but modern motherboards often share lanes between slots, which can limit bandwidth. This is especially true in cases where there are multiple high-speed M.2 slots vying for the same PCIe lanes, leaving the available bandwidth unknown to the user. To prevent any surprises, it is crucial to thoroughly read your motherboard's manual and understand how its PCIe lanes are distributed before committing to a particular expansion card.

For example, the MSI MPG X870E Carbon Wi-Fi motherboard shares its primary PCIe slot, housing the GPU, with the second PCIe slot and the M2_2 connector. This results in the RTX 4090 running at PCIe 4.0 x8 instead of the ideal PCIe 4.0 x16, cutting its available PCIe bandwidth in half. However, this issue does not arise when utilizing the third PCIe slot, as it is not connected to the CPU lanes.

Even more old motherboards, like the Asus ROG Strix X570-E, also suffer from similar restrictions. Physically larger slots may still have limited lanes, such as PCIe 5.0 x4 on the second slot of the MSI X870E, while PCIe 4.0 x4 through the chipset on the third slot. Despite this, the secondary slots may not provide sufficient bandwidth for high-end PCIe expansion cards, like the Asus Hyper M.2 x16, which requires four NVMe drives.

Overall, dropping from PCIe 5.0 x16 to x8 or even x8 might not drastically impact gaming performance, as demonstrated by a 1-4% performance difference in certain scenarios. However, it is essential to consider the motherboard's PCIe specifications, as older motherboards with PCIe 3.0 slots may experience more noticeable performance drops when transferring data between RAM and VRAM. Always read your motherboard manual to ensure you are making an informed decision before purchasing and installing an expansion card.