External GPUs explained: What they are and what they do

Summary

• External GPUs connect to laptops and gaming handhelds via Thunderbolt or other connectors, giving you more power when you don't need portability.
• Bandwidth limitations with Thunderbolt can impact the performance of external GPUs.
• External GPU enclosures allow for easier upgrades, but may come with a high price tag and size limitations.

Since PC gaming handhelds started to become more prominent, the idea of external GPUs has gained some popularity. Because gaming handhelds are heavily restricted in terms of power (so that they can be portable), the idea of adding more power on demand when you're stationary makes sense.

But GPUs usually go inside a desktop computer, how can you connect them to a laptop or gaming handheld? How does an external GPU work in order to provide this boost in power? Let's take a closer look.

How external GPUs work

They're just GPUs... but outside your PC

A GPU, or graphics card, is a component of your computer that's responsible for rendering the image you see on your PC, which is particularly important in games since they have very complex geometry and textures. A GPU usually communicates with your PC using a PCIe x16 connection. Desktop PCs have these long PCIe (which stands for Peripheral Component Interconnect Express, by the way) slots, which are designed to fit a graphics card. The x16 here refers to the fact that the GPU uses 16 lanes of communication to send and receive data from the motherboard.

Typically, you can't connect PCIe devices to a computer from outside the chassis, since they need to directly link to the motherboard. However, thanks to technology like Intel's Thunderbolt, it's become possible to connect external PCIe devices to a computer, most notably laptops and gaming handhelds. More recently, we've also seen the rise of competing technologies, like OCuLink and some proprietary solutions from companies like Asus. These all serve the same purpose: letting you use external GPUs (and super-fast SSDs) with your PC.

Essentially, in an external GPU setup, you'll have a normal GPU connected to a board that then routes traffic to the Thunderbolt port to be sent to the laptop, so it works much like it would if it was inside your PC.

Limitations of external GPUs

There's a bandwidth problem

This all sounds great on paper, but there's something that holds external GPUs back very significantly, and that's bandwidth. As we mentioned above, a typical desktop GPU operates on a PCIe x16 connection, with the most high-end GPUs all using PCIe 4.0. A PCIe 4.0 x16 connection can offer up to 31.5GB/s (that's gigabytes, not gigabits) of bandwidth, meaning data moves incredibly fast between your GPU and motherboard.

Thunderbolt 4, however, only supports PCIe 3.0, and at that, it only has four PCIe lanes, so the total bandwidth caps off at 40Gbps, which is equivalent to around 5GB/s. That's a huge downgrade compared to full PCIe connection, and most modern GPUs will be severely hampered by this limited bandwidth, especially as game assets get bigger and bigger.

Some rival technologies have arisen as a result. OCuLink is one that's been rising in prominence, since it's a fairly open standard anyone can use. The latest version of OCuLink supports PCIe 4.0 and it has a total of eight lanes, so the total bandwidth goes up to 16GB/s, far closer to what you can get with an internal GPU. And yet, it's still only halfway there, so you might still see a big performance hit.

It's worth noting that Thunderbolt 5 is starting to hit the market too, doubling the bandwidth from Thunderbolt 4, plus adding the option to triple the bandwidth in a single direction. This should enable far more capable external GPUs compared to Thunderbolt 4, though OCuLink is still better.

External GPU enclosures and all-in-one solutions

Know what you're getting

If you're on the market for an external GPU, it's important to know that there are different kinds of solutions out there. Traditionally, we've mostly had external GPU enclosures. These are large shells where you can install a full desktop GPU as if it was a proper computer. You then connect the external GPU chassis to your laptop, and you're good to go.

External GPU enclosures are beneficial in that they let you use full-blown desktop GPUs and they can, in theory, be upgraded over time by just buying the GPU itself and swapping it out. They're a better long-term investment on paper. Unfortunately, there haven't been a ton of new external GPU enclosures coming out recently, and recent graphics cards have become huge, so it's impossible to fit many modern cards in one of these enclosures.

It's also worth noting that these enclosures can be pretty expensive by themselves, costing upwards of $300. That might make it seem like you're getting an actual GPU, but it's just the enclosure, so be careful not to buy one expecting to have a full setup out of the box.

Since PC gaming handhelds started becoming more popular, another common option is to buy a fully integrated external GPU. This includes the enclosure and the GPU itself all integrated in one package. This makes setup a bit easier, and it also means that, in many cases, the GPU can be a laptop version, rather than a full desktop one, making the overall package much more portable and easier to carry.

The downside is that laptop GPUs aren't as powerful, and you're also sacrificing upgradability, but that may not be a huge problem considering the bandwidth limitations of Thunderbolt and the fact that many external GPU enclosures can't fit modern high-end GPUs anyway.

Should you buy an external GPU?

It's not for everyone

An external GPU can be a great way to get more performance out of a laptop, gaming handheld, or even a mini PC without committing to a large desktop case you can't take anywhere. However, external GPUs cost hundreds of dollars on top of your laptop, and many times, cheaper laptops won't be compatible with them, so you need to make a big investment.

On the flip side, that external GPU will likely work with future laptops you buy and still offer better performance (until integrated graphics surpass it), so it can be a good investment. And if you have a discrete enclosure, you may even be able to upgrade the GPU alone without spending extra on a new enclosure, so that cost isn't as high as it could be otherwise.

Still, not everyone really needs this kind of power, especially if it means being tethered to an outlet. At the end of the day, it's up to your personal needs and budget.