Key findings
- The number of PCIe lanes available for the GPU and primary NVMe SSD varies between AMD and Intel CPUs.
- Using an NVMe SSD in the primary M.2 slot halves the GPU bandwidth on Intel CPUs on many motherboards.
- AMD CPUs perform better in this regard because they provide enough PCIe lanes for the GPU and NVMe SSD to avoid halving the bandwidth.
- The drop in GPU performance with 8 vs. 16 PCIe lanes remains negligible.
The concerns about bandwidth sharing between the PCIe x16 slot and the primary M.2 slot aren't new – PC builders have been concerned about PCIe lane sharing for at least 10 years, if not longer. When I read up online to get to the bottom of this issue, I found too many conflicting reports and opinions. People just don't seem to agree on whether or not an NVMe SSD really “slows down” a graphics card.
While there is enough documentation from CPU and motherboard manufacturers that this is not a problem, it turns out that the flood of information from the community overwhelms anyone, especially new PC builders. I will explain it in the simplest terms possible so you know what to look for when buying a new motherboard.
Intel and AMD behave differently
It is due to the different approaches to PCIe 5.0
So let's start with how your processor communicates with components like graphics cards, NVMe SSDs, and other less resource-intensive hardware. Every motherboard has a fixed number of PCIe lanes distributed between those connected directly to the CPU and those connected to the chipset.
It is always advisable to connect high-speed components such as graphics cards and fast NVMe drives to the slots that have the most direct-to-CPU PCIe lanes.
Depending on which PCIe, M.2 and other slots you use to connect your components (and in what combination), you can use either the lanes that go directly to the CPU or the chipset. Ideally, you should use the ones that have a direct connection to the CPU to minimize latency. Therefore, it is always advisable to connect high-speed components such as graphics cards and fast NVMe drives to the slots that have the most PCIe lanes with a direct connection to the CPU.
These slots are almost always the PCIe x16 slot (usually the top one) for the graphics card and the primary M.2 slot (usually labeled M.2_1) for your fastest NVMe SSD. You could put the graphics card and primary SSD in each of these two slots and be done with it, but depending on your processor and motherboard it gets a little tricky; bear with me.
Intel's 13th and 14th generation CPUs have 16 PCIe 5.0 lanes shared between the primary PCIe and M.2 slots; AMD CPUs have 20.
Both Intel and AMD CPUs have a total of 28 PCIe lanes coming from the CPU, but their composition is different. Intel's 13th and 14th generation CPUs have 16 PCIe 5.0 lanes shared between the primary PCIe and M.2 slots, and 8 PCIe 4.0 lanes for the chipset. AMD's Ryzen 7000 CPUs, on the other hand, have 20 PCIe 5.0 lanes shared between the primary PCIe and M.2 slots, and 8 PCIe 5.0 lanes for the chipset and other connections.
Your NVMe SSD could halve your GPU bandwidth
It still depends on your motherboard chipset.
For an Intel CPU, installing an NVMe SSD (any generation) in the primary M.2 slot can cut the number of PCIe lanes for your graphics card in half. With an unused M.2_1 slot, instead of your GPU using all 16 available PCIe 5.0 lanes, it now only has 8 lanes available and 4 lanes are used by the SSD. Due to the nature of the PCIe lanes split, a 12x/4x split is not possible.
To avoid any surprises, you should read the motherboard manual before purchasing it.
However, this scenario only occurs on Z790 motherboards with a PCIe 5.0 M.2 slot and not on all Intel motherboards. You will need to read the motherboard manual for the specific model you are planning to purchase so you are not caught off guard. Even on affected motherboards, using a secondary M.2 slot that uses PCIe 4.0 instead can get around this issue.
And as you'd expect, AMD processors allow your graphics card and primary SSD to utilize 16 and 4 PCIe lanes, respectively—no 8x/4x split required. However, only the X670E chipset guarantees PCIe 5.0 lanes for your GPU; other chipsets may still only offer PCIe 4.0 support for the graphics card. AMD's Ryzen 9000 processors also continue to offer this support, but this time on the X870 and X870E chipsets.
If you are using a PCIe 4.0 SSD, you can skip the PCIe 5.0 M.2 slot entirely and use one of the other slots that are directly connected to the CPU.
So, to sum up, AMD processors are, at least in theory, better suited for users who want to use a high-end GPU and a PCIe 5.0 SSD together without sacrificing bandwidth for either.
However, if you only have a PCIe 4.0 SSD, you can skip the primary PCIe 5.0 M.2 slot on your Intel motherboard (and AMD, of course) altogether and use one of the PCIe 4.0 slots connected directly to the CPU. This way, you don't have to worry about sharing lanes between your SSD and GPU. The motherboard manual can help you find the right slot and guide you in your purchasing decision.
Why you still won’t notice the difference
The performance reduction is negligible
With all that out of the way, the difference in GPU performance between 16 and 8 PCIe lanes won't be noticeable to most users. First, there are no graphics cards that support PCIe 5.0, so your GPU will be running on PCIe 4.0 anyway, even in a PCIe 5.0 slot. Second, even if you put an SSD in the primary M.2 slot and your graphics card has 8 lanes available instead of 16, the performance drop in gaming is negligible – around 1-3%.
In case you were worried about this, you are not leaving any power unused.
Graphics cards are still a long way from maxing out the full PCIe 4.0 x16 connection, so reducing it to x8 won't affect your performance as much as you might think. It's only if you're running workstation-level systems that you're likely to need the most bandwidth, but in that case you'll probably need workstation CPUs like AMD's Threadripper chips and others.
PCIe 5.0 is still a relatively new standard, at least for GPUs, so it will be years before GPUs require the full bandwidth of PCIe 5.0. In fact, they are not even close to the full bandwidth of PCIe 4.0.