A motherboard will consist of many PCIE slots, and it’s your job to use them wisely to get the most performance out of your system.
Not all PCIE slots perform the same, some use different lanes, some have more bandwidth, and some have less latency.
So in a nutshell, it absolutely matters what PCIE slot you use for your graphics card, using the wrong slot WILL bottleneck the graphics card.
Why Does PCIE Slot Matter For GPU
GPUs are made to insert into x16 slots, but not all x16 slots are the same. Some x16 slots will run at x8 mode, or even x4 mode, your goal is to ensure it runs at x16 mode.
The most optimal slot for a graphics card is the x16 slot because it offers the most bandwidth, this is important because the GPU is a device that moves a TON of data.
This is why we always ask users to install their graphics card in the TOP PCIE slot because it’s hardwired to use 16 PCIE lanes.
| Version | x1 Throughput | x2 Throughput | x4 Throughput | x8 Throughput | x16 Throughput |
|---|---|---|---|---|---|
| 1.0 | 0.250 GB/s | 0.500 GB/s | 1.000 GB/s | 2.000 GB/s | 4.000 GB/s |
| 2.0 | 0.500 GB/s | 1.000 GB/s | 2.000 GB/s | 4.000 GB/s | 8.000 GB/s |
| 3.0 | 0.985 GB/s | 1.969 GB/s | 3.938 GB/s | 7.877 GB/s | 15.754 GB/s |
| 4.0 | 1.969 GB/s | 3.938 GB/s | 7.877 GB/s | 15.754 GB/s | 31.508 GB/s |
| 5.0 | 3.938 GB/s | 7.877 GB/s | 15.754 GB/s | 31.508 GB/s | 63.015 GB/s |
| 6.0 | 7.563 GB/s | 15.125 GB/s | 30.250 GB/s | 60.500 GB/s | 121.000 GB/s |
So as you can see, the x16 slot has double the bandwidth as the x8 slot, this is because it has twice as many PCIE lanes.
And with each PCIE generation, the bandwidth doubles, so using the newest PCIE generation can be a huge benefit for your graphics card.
PCIE x4 Vs x8 Vs x16 Average FPS
The x16 slot at x16 mode generates more FPS on average compared to all the other PCIE slots, this is because of the extra bandwidth.
The PCIE x4 slot generated an average FPS of 110 across 11 games, this is significantly behind x8 and x16.
The PCIE x8 slot generated an average FPS of 120 across 11 games, this is a huge jump compared to the x4 slot.
The PCIE x16 slot generated an average FPS of 124 across 11 games, this is only a 4 FPS jump compared to the x8 slot whereas the jump between x4 and x8 was 10 FPS.
Also Read: Does Motherboard Affect Performance?
So What’s The Right PCIE Slot For My GPU?
The top PCIE slot is the best PCIE slot for the GPU for two reasons. The top PCIE slot is prioritized by the CPU, and it’s always running at x16 mode.
The bottom PCIE slot isn’t the best for your graphics card despite looking the same as the top PCIE slot, this is because it actually runs at x8 mode.
It actually has x8 electrical PCIE lanes that run to the CPU, the rest aren’t actually electrical therefore the second slot is capped at x8 mode.
But always consult your motherboard manufacturer, or the manual for the right information, this is because not all motherboards operate the same.
CPU Lanes Vs Chipset Lanes For GPU
If you didn’t know already, there are two types of PCIE lanes present inside your PC, some lanes are run by the CPU, and some are run by the chipset.
The CPU lanes are the lanes you should always use for your GPU, this is because it has more bandwidth, and less latency compared to chipset lanes.
The CPU will usually allocate x16 lanes for the graphics card, and 4 lanes for M.2 drives because these devices are usually high bandwidth.
The chipset allocates lanes which are typically shared between USB, Ethernet, M.2, Audio, Firmware, and SATA. These devices are low bandwidth so chipset lanes are perfect for them.
Chipset’s Limited Bandwidth Can Hurt GPU’s
The chipset lanes will naturally have a 4-lane bus connection to the CPU which is huge because the lane is usually 4 lanes wide.
This means high performing devices such as the GPU will be severely bottlenecked to 4 lanes, you may lose a considerable amount of FPS due to this.
And the loss of FPS may be amplified due to the fact that the chipset lanes are constantly being used by low bandwidth devices such as USB, Audio, and SATA.
This is why it’s important to ensure that your CPU has at least x16 PCIE lanes on it, and you ALWAYS use the top PCIE slot as that’s always prioritized by the CPU.
Graphics Card SLI & Crossfire Impact On Performance
When you use Crossfire or SLI, the second x16 PCIE slot will be capped at x8, this is because CPUs have a limited number of PCIE lanes which will need to be shared between two graphics cards.
When you have one graphics card, your CPU can easily dedicate x16 lanes to one card, when you add another card, your CPU will have to share the x16 lane which equates to x8 lanes per card.
Luckily, a x16 slot running at x8 mode doesn’t exactly equate to half the amount of FPS despite having half the amount of bandwidth.
You will only lose a few frames at most compared to running a graphics card in the full x16 slot. So SLI or Crossfire, you really aren’t losing a ton of FPS due to the cards running in x8 mode.
Conclusion
In conclusion, it does matter what PCIE slot you use for gaming, different slots operate at different bandwidths, and latencies which can affect how the GPU performs in graphically intensive operations.
A lot of the confusion comes from the fact that the PCIE slot and the PCIE lanes are named the same, so even if you have a x16 slot, it doesn’t mean it runs at x16 mode.
