High-end external GPUs still suffer a performance hit — OCuLink tests show up to a 23% drop with an RTX 4090 | Tom’s Hardware

An external GPU (eGPU) lets you use the best graphics cards with slim and compact devices that lack graphics firepower. But it continues to be the case that you don’t get the full graphics card performance, due to connection limitations. One Weibo user recently evaluated the performance hit that you take from using the OCuLink interface. For those not in the know, OCuLink is an optical and copper link first developed for PCIe connections more than a decade ago, which has mostly been used in the server realm. But it’s been getting more attention on the consumer side lately, with products like GPD’s Win Max 2.

eGPUs, essentially graphics cards housed inside a box or installed on a dock, have been around for ages. You can connect them to various devices, such as laptops, handheld gaming consoles, iMacs, or mini-PCs. Thunderbolt has typically been the preferred interface to connect eGPUs to other devices, but it’s also expensive. Renewed interest in OCuLink has provided manufacturers with an alternate option to establish communication with the host device and the graphics card.

One of OCuLink’s most appealing features is the higher bandwidth, meaning there is more room for the graphics card to stretch its legs. Thunderbolt 4 is limited to 40 Gb/s, whereas OCuLink enables a bandwidth of up to 64 Gb/s. That’s a whopping 60% increase. So theoretically at least, you should get more performance over an OCuLink connection. But, Intel has already announced Thunderbolt 5, which is rated for up to 80 Gb/s in standard mode.

The Weibo user connected his dock via OCuLink to a laptop powered by the latest Intel Core Ultra 125H (Meteor Lake) processor. While he compares the performance to a desktop, he doesn’t reveal the PC’s specifications. So approach these performance numbers cautiously;  the reviewer is comparing a 28W chip to a much more powerful desktop processor. A chip from Intel’s Raptor Lake Refresh HX series or AMD’s Ryzen 7045 series (Dragon Range) would be a fairer comparison.

OCuLink Benchmarks

Starting with the flagship GeForce RTX 4090 and the Time Spy benchmark, there was a 23% performance loss when using the internal display. However, with an external display, the performance hit was 17%. The performance penalty was lower with the Time Spy Extreme benchmark. The reviewer observed 5% lower performance with the internal display and zero performance loss with an external display. On the other hand, the GeForce RTX 4070 Ti Super only lost 10% of its performance when using the internal display.

So the results clearly show that using an external display minimizes the performance drop, because the data travels directly to the external display instead of having to return to the laptop — this frees up bandwidth. It would be interesting to see the same tests with a more capable mobile processor. The Time Spy Extreme test with the GeForce RTX 4090 and the external display, which came back with zero performance hit, suggests a processor bottleneck.

Zhiye Liu is a Freelance News Writer at Tom’s Hardware US. Although he loves everything that’s hardware, he has a soft spot for CPUs, GPUs, and RAM.