Is Your GPU Running At x4 (or x8) PCIe Speeds By Mistake? (Is x16 Actually Needed?)

The PCIe slots on motherboards can be a bit confusing. It would be SIMPLE if they were all full length x16 slots that each gave full x16 speeds, but unfortunately they do not. Some PCIe slots are full length, but only run at x4. Other PCIe slots DO run at x16… but only if you avoid plugging NVMe drives into certain M.2 connectors.

Equally some BIOS settings exist that can make your main PCIe x16 slot run at x8 or even x4 speeds. So in this video I discuss how to check what your graphics card is running at, and then I cover the three reasons why GPUs can run at less than x16 speeds. Finally I look at whether this actually matters or not.

If you prefer text over video, please read on for the guide/transcript version of this video.

Video Transcript And Guide

Is Your GPU Running At x4 Or x8?

Modern gaming motherboard
Modern gaming motherboard

Hey everyone, the PCIe slots on motherboards can be a bit confusing. After all, if the board has two “full size” x16 slots, you might think that you can plug your graphics card into EITHER of these slots – but you’d be wrong. You might even be running your GPU at just a QUARTER of its intended speeds, reducing your gaming performance. What am I talking about? Well if you go into the detailed specs of most motherboards, you’ll see that they say things like “PCIe x16 slot COMMA supports x4 speed”. OR some x16 slots might only run at x8 speeds if you plug in an M.2 NVMe drive in a specific slot.

This can be BAD because some graphics cards NEED to run at their full bandwidth – or certainly at more than x4 speeds. So why might your GPU be running slower than the x16 speeds you were expecting? And why do these PCIe slots have so many confusing subclauses?

How To Check This

GPU Z screenshot showing the bus interface PCIe section
GPU Z screenshot showing the bus interface PCIe section

Well let me start off by discussing HOW to check the actual speed your GPU is running at. The best approach is to download GPU-Z which is a really useful (and free) bit of software. You can download it from the TechPowerup site and either install it or run it as a standalone program. Once it’s opened, look at the “Bus Interface” section over on the right. This box might seem a bit confusing but it actually tells us two really important things: the number with a decimal point tells us what generation of PCIe the card is running at – you’ll hopefully see 3.0, 4.0 or 5.0 here although really old PCs might still have gen 2 cards I guess. The other number (with the “x” next to it) is how many lanes of PCIe bandwidth your card is running at. If you plugged your GPU into a “full length” PCIe port on your motherboard, you would HOPE that this number here is x16. But if it’s only showing x4 or x8, what gives? Is something WRONG with your motherboard or GPU?

PCIe Lanes Breakdown

Well, probably not. There’s three main reasons why your GPU might actually be running at x4 or x8 speeds instead of the full x16 speeds, and this all comes back to how motherboards are designed, and secondly how PCIe bandwidth is “distributed”, for want of a better word.

Think of PCIe lanes as roads, allowing cars (or data) to flow through them. The more lanes, the better. You’ll often have 20 lanes or so that route directly to the CPU – this breaks down as 16 lanes for the graphics card, and 4 lanes for a primary NVMe SSD. But of course, us consumers need MORE PCIe bandwidth than that, right?

intel 10gbe card
intel 10gbe card

We might want a WiFi or 10 Gig Ethernet card, as well as a second NVMe drive so that we can install Black Ops 6. I still can’t believe how big games are getting. Anywhoo, how should all this extra PCIe bandwidth be delivered? Well workstation-grade CPUs like Threaddripper support 64 to 128 PCIe lanes (which is LOADS) – but sadly consumer CPUs are stuck at 20 or so lanes.

Intel webpage showing 20 PCIe lanes total
Intel webpage showing 20 PCIe lanes total

That means that the motherboard chipset is left to pick up the slack, and so all the other stuff – so WiFi cards and second NVMe drives – will often be handled through the chipset. That’s why you see motherboard specs split up the different PCIe lanes into the CPU and chipset, and essentially the chipset lanes are bottlenecked a bit because of how they connect to the CPU PCIe link.

Is Your GPU In The Wrong Slot?

Mobo slots PCIe
Mobo slots PCIe

Right, so technical overload aside, how does this relate to our GPUs running at the wrong speeds? Well the FIRST reason this might be happening is that if a motherboard has multiple “full size” PCIe slots, only the TOP one is routed directly to the CPU’s PCIe lanes. The rest tend to be routed via the chipset. As a result if you have plugged your GPU into one of the bottom PCIe slots, it’s likely to be running at x4 speeds and you’ll just need to move it. The reason this happens is that even though the motherboard is offering you a full length PCIe slot, only 4 of the “pins” are connected up electrically on the circuit board – due to chipset PCIe limitations.

Reduced PCIe Bandwidth Due To NVMe

The SECOND reason that this might be happening is that some motherboards will reduce the PCIe bandwidth if you use certain M.2 NVMe slots. For example let’s look at the Gigabyte Z790 Aorus Pro X: this amazing motherboard has THREE full-length PCIe slots and it has FIVE M.2 connectors – which is great.

Z790 AORUS Gigabyte motherboard
Z790 AORUS Gigabyte motherboard

HOWEVER if you look at the technical specs for this motherboard, you’ll see that it says that the main PCIe gen 5 slot “shares bandwidth with the M2C_CPU connector” and that slot “operates at up to x8 mode when a device is installed” in that M2C slot:

subclause 1
Gigabyte motherboard spec page saying that the main X16 slot can run at just x8 speeds.

So in other words, if you put an M.2 drive into this “C” slot, it will reduce your main graphics card’s bandwidth down to x8 speeds. Of course this is still gen 5 we’re talking about so that’s unlikely to harm your gaming performance much, but I’ll circle back to this point in a minute.

BIOS Bifurcation

The THIRD reason why your GPU might be running at x4 or x8 speeds is that you have BIOS bifurcation turned on. I hate saying that word, I usually mispronounce it – so I’m glad I said it right! This setting allows your motherboard to split the x16 lane of the main PCIe slot into two x8 lanes instead (or sometimes four x4 lanes). This can then allow for more PCIe devices to be plugged in effectively. While this setting IS usually disabled by default, I would still dig around inside your BIOS for the bifurcation setting just to confirm that it IS set to the x16 option.

BIOS bifurcation setting on my ASUS motherboard
BIOS bifurcation setting on my ASUS motherboard

Does It Actually Matter?

Okay, so that explains WHY a GPU might be running at less than x16 speeds: now the big question – does it matter? Well I think there was a time in the past when this WOULD have mattered – PCIe gen 3 bandwidths weren’t too high, for example, and so if your GPU was accidentally running at x4 speeds, you’d be limited to 4 Gigabytes a second, which might be insufficient for some modern games. However PCIe gen 4 (and certainly gen 5) are so fast that a gen 4 card running at x8 (for example) will probably make ZERO real world difference to you. YES maybe if you did loads of benchmarking you would notice a small difference in FPS, but in reality it wouldn’t be a major difference.

Odin Hardware has done some great round-up videos showing that with PCIe gen 3, you do see some performance drop off at x4 speeds but there’s THEN no MAJOR difference between x8 and x16 speeds. And remember that’s at gen 3, when bandwidth is a bit lower across the board. When you move up to gen 4, there’s much less difference to a GPU running in the x4 slot – YES there is still an FPS drop-off, but it’s not super substantial. And the x8 frame rates are actually very similar to the x16 ones.

Odin Hardware FPS tests with a PCIe gen 3 GPU
Odin Hardware FPS tests with a PCIe gen 3 GPU

So as you can see, you’re not going to have major issues if your GPU is somehow running at x8 speeds due to where you installed your M.2 drive (for example). And it’s good to know: that your gaming performance won’t be massively crippled if your GPU is stuck at x4 speeds, but clearly this isn’t an ideal configuration and that’s why motherboard makers (in the manual) recommend that you try to put your primary GPU in the top PCIe slot.

So if you’ve gone out and you’ve already purchased your motherboard and you’re now worried after watching this video that maybe your new build, your shiny new build is going to run super slow and your gaming performance will be crippled… please don’t worry too much. As you can see from this video, in many cases, even if your graphics card is running x8 (or even x4 speeds in some cases) it’s not going to make a massive real world difference to your gaming performance. 

It would matter if you’ve got some ten year old builds, and your graphics card has been running x4 for the entire time, but then you’re probably used to having slightly laggy gaming performance, especially with today’s overblown, bloated computer games!

cropped A picture of me Tristan
About Tristan Perry

Tristan has been interested in computer hardware and software since he was 10 years old. He has built loads of computers over the years, along with installing, modifying and writing software (he's a backend software developer 'by trade').

Tristan also has an academic background in technology (in Math and Computer Science), so he enjoys drilling into the deeper aspects of technology.

Tristan is also an avid PC gamer, with FFX and Rocket League being his favorite games.

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