Can a Motherboard Bottleneck a GPU?

Yes, in short, a motherboard CAN bottleneck a GPU, however, the exact cause of the bottleneck isn’t the entire motherboard, but a specific interface called PCIe and the corresponding PCIe slots.

There are two ways the motherboard can bottleneck your GPU. Firstly, if you have a newer generation graphics card and you plug it into an older generation PCIe slot, then your GPU can get bottlenecked. Secondly, if you install the GPU on a slot with fewer than ideal lanes, then your GPU can get bottlenecked.

The bottleneck is often marginal particularly for gaming and is often seen on high end graphics card. Low end or mid range graphics cards do not really suffer from bottlenecks since their throughput rate is much lower and thus does not saturate the bandwidth of the PCIe slots.

In the following text I will attempt to answer the question “can a motherboard bottleneck a GPU?” and how to avoid bottlenecks

Can a Motherboard Bottleneck a GPU?

There is often a misconception that the motherboard itself doesn’t affect the performance of a graphics card.

That is not true. A motherboard is nothing but an assembly of several important sub components jammed up into one place. One such important component is the PCIe interface.

Also Read: How to Know if Graphics Card is Compatible With Your Motherboard?

A Primer on PCIe Interface

The PCIe interface comprises of PCIe lanes that act as communication highways connecting the various PCIe devices such as graphics card, WiFi cards, NVMe SSDs etc to the CPU.

A graphics card is effectively a PCIe device and PCIe slots are where you connect PCIe devices to.

There are various sizes to the PCIe slots i.e x1, x4, x8 and x16. The number after the ‘x’ often denotes the amount of PCIe Lanes the slot offers.

So an x1 slot offers one PCIe lane and an x8 slot offers 8 PCIe lanes.

does it matter which pcie slot I use w
PCIe Slots for adding expansion cards. Note that the bottom x16 slot actually only has 4 lanes. You can figure this out by reading the specsheet of the motherboard.

An x16 slot is the largest slot and it also the most confusing since often many times you may come across an x16 slot that has the size of the full x16 slot, but may only offer 8 or even 4 lanes.

Now a graphics card ideally requires an x16 slot or 16 lanes. It can however work on an x8 slot as well but that may introduce bottlenecks.

Also Read: How Many PCIe Lanes Does a GPU Use?

Different Version of the PCIe Interface

Now there are different versions of the PCIe interface. With each newer version, the per lane speed of the PCIe interface doubles.

So for instance, a x1 slot (or one lane) on a PCIe 3.0 has speeds of 0.985 GB/s. The same on a PCIe 4.0 has speeds of 1.969 GB/s.

Newer PCIe interface entails that the slot can be plugged in with more demanding graphics cards that generate a lot more data.

The following chart shows the various speeds of the different slots on different PCIe versions.

Versionx1
(GB/s)
x2
(GB/s)
x4
(GB/s)
x8
(GB/s)
x16
(GB/s)
1.00.2500.500 1.0002.0004.000
2.00.5001.0002.0004.0008.000
3.00.9851.9693.9387.87715.754
4.01.9693.9387.87715.75431.508
5.03.9387.87715.75431.50863.015
6.07.87715.75431.50863.015126.031

For the purpose of this article, you need to remember the following:

  • There are various sizes of the PCIe slot and the lane count for each slot can vary.
  • A GPU ideally requires an x16 slot (16 PCIe Lanes).
  • There are different versions of the PCIe interface. The most current is PCIe 4.0.

Also Read: What are PCIe Lanes?

So How Can Motherboard Bottleneck a GPU?

With the primer on PCIe lanes, you should be better able to grasp the reasons explained below.

There are two reasons how a motherboard can potentially bottleneck your GPU.

  1. If You Plug Your GPU in a PCIe Slot With Fewer than 16 Lanes
  2. If You Plug a Newer Gen GPU in an Older Gen PCIe Slot

1. If You Plug Your GPU in a PCIe Slot With Fewer than 16 Lanes

As mentioned, a Graphics Card ideally requires an x16 slot. Meaning it requires 16 lanes for best performance.

If you were to install a graphics card on any other slot i.e x8 slot, then there is a possibility that your GPU can get bottlenecked.

A motherboard often has multiple x16 slots. However, not all x16 slots are the same as some x16 slots may only offer 8 or 4 lanes in reality.

As a rule of a thumb, the first x16 slot on a motherboard almost always offers the full 16 lanes. As such, this is the most ideal slot for installing a GPU.

The second or third x16 slots on a typical motherboard offer 8 or 4 lanes.

Take the following motherboard for instance.

can a motherboard bottleneck GPU
Image: MSI MPG Z490 Gaming Carbon WiFi

This motherboard has three PCIe x16 slots. However, only the top offers the 16 lanes.

The bottom x16 slot only offers 4 lanes, hence can cause serious bottlenecks for your GPU.

The second x16 slot (middle) is a bit interesting. This PCIe slot shares the lanes with the top x16 slot. Meaning, IF the middle slot is occupied both the top and the middle slot will divide their lanes by half. So both the the top and the middle slot will have 8 lanes. 

The middle slot is often used when you have multiple GPUs. If you have a single GPU, always use the top x16 slot and leave the middle slot empty.

Here is how the configuration will look like

  • Single GPU: x16/0/x4
  • Multie GPU: x8/x8/x4

It is of paramount importance that you must always refer to the motherboard specsheet in order to figure out how many lanes a certain PCIe slot actually connects to.

Does Installing a GPU in an x8 Slot Bottleneck It?

That depends upon two things: the amount of data your GPU generates and the application you are using.

If you have low-mid range graphics card that does not generate a lot of data, and therefore, does not saturate the bandwidth provided by 8 lanes, then the GPU will not get bottlenecked.

On the other hand, if you have a high end GPU that does generate more data than the bandwidth provided by 8 lanes, then it can get bottlenecked.

There is a study by Pudget Systems I would like to mention here.

Pudget Systems Performance Report on Using x8 or x16 Slots of GPU

Pudget Systems has conducted a great study on this topic. They tested an NVIDIA Titan X PCIe 3.0 device on an x8 and an x16 slot while running different software and games.

The results are fairly conclusive in that a high end GPU does suffer from performance bottlenecks when installed on an x8 slot.

On certain applications the differences are quite marginal, while in others it is quite significant.

For instance, when using only a single graphics card on either an x8 or on an x16 slot, the performance bottleneck is only marginal.

Ashes of Singularity x8 x16
Ashes of Singularity suffers little from performance bottleneck. Source: Pudget Systems

However, when using two graphics card in x16x16 vs x8/x8 configuration, the performance bottleneck with x8/x8 is as much as 30% on certain applications like when running Unigine Heaven Pro 4.0.

Unique Engine 4.0
Source: Pudget Systems. At 4K, Unique Heaven Pro 4.0 shows an impact of 30% comparing x8/x8 vs x16/x16 configuration.

Again, the impact isn’t TRUE for ALL applications and games.

In fact, games generally do not suffer much from this bottleneck as much as software do.

However, to keep things ideal and the performance drop minimal, always install the GPU on an x16 slot on your motherboard.

If your motherboard does not have a working x16 slot, then unfortunately, your motherboard would indirectly bottleneck your GPU if it is installed on any other slot.

Also Read: How to Check If PCIe Slot is Bad?

2. If You Plug a Newer Gen GPU in an Older Gen PCIe Slot

Installing a newer gen CPU in an older Gen PCIe slot can also introduce bottlenecks.

For instance, if you have PCIe 4.0 graphics card such as the NVIDIA RTX 3090 and if you were to plug it into your old motherboard which conform to PCIe 3.0 interface, then you can introduce performance bottlenecks.

Thankfully, Pudget Systems has also tested the bottlenecks a newer GPU would be subjected to if used on an older PCIe slot.

Davinci Resolve PCIe testing

Image: PudgetSystems.com. Comparison of how RTX 3090 and Titan RTX perform on different PCIe generations. A significant impact can be seen on performance in DaVinci Resolve (video editing software) when using the cards in older PCIe slots.

Pudget System tested a PCIe 3.0 Titan RTX and a PCIe 4.0 RTX 3090 on PCIe 1.0, PCIe 2.0, PCIe 3.0 and PCIe 4.0 x16 slots.

As expected, the PCIe 4.0 RTX 3090, gives the best performance on a PCIe 4.0 slot and suffers in score when used in an older PCIe 3.0 slot (156 vs 150 points).

Therefore, if you have a newer gen device but an older motherboard with older gen PCIe slots, then only way to get the max performance is to upgrade your motherboard.

Also Read: When to Upgrade Motherboard?

Note that it is possible for the same motherboard to feature certain slots conforming to a newer PCIe version while others conforming to an older PCIe version.

Take for instance, the AMD B550 motherboards. Here certain slots conform  to PCIe 4.0 while others connect to PCIe 3.0.

Asus Strix B550-F Gaming
Asus Strix B550-F Gaming. Top slot conform to PCIe 4.0. The rest conform to V3.0. For GPU would choose the slot conforming to the newest PCIe version.

Also Read: Does Graphics Card Affect FPS?

Summary

Can a motherboard bottleneck a GPU? Yes it can and this is how:

  1. If it does not have a free or a working x16 slot with full 16 lanes, then a GPU can get bottlenecked if installed in a slot with fewer than 16 lanes.
  2. If the motherboard’s PCIe slot conforms to older PCIe version as compared to the GPU, then a GPU can get bottlenecked.

In closing remarks, I cannot stress the fact that the motherboard specsheet is your best friend here. Always check the specsheet for figuring out the PCIe slot that offers the full 16 lanes and conforms to the newest PCIe version.

Also Read: Can B Series Motherboards Overclock?

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Author:

Atif Qazi
Atif Qazi is the founder of PCGuide101. He is a digital nomad who loves everything PC. He is a PC builder, tech enthusiast, engineer, and a lover of single player lore-rich RPG games.

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