Getting There...

Overall results are very interesting for this new eGPU + ultraportable combination. With the GTX 1080 and Core i7-8550U in hand, across the games we tested there was a 61 percent performance improvement on average compared to the GTX 1070 and Core i5-7200U combo we looked at earlier. A lot of games that were previously unplayable at maximum settings are now playable, plus stuttering was less of an issue as evidenced by far more titles with 1% lows above 30 FPS.

This is also a much larger gain than we'd normally see when comparing the GTX 1080 and GTX 1070. With a typical desktop or laptop system with the same CPU, the GTX 1080 is only 20 to 30 percent faster than the GTX 1070, but here we're seeing a 60 percent average gain. It's quite clear that the additional CPU power helps to improve performance in a significant way, and those with older dual-core laptops won't necessarily be able to achieve large performance gains simply by pumping in more GPU power.

While for the most part results are really good, and every game we tested is very playable at high quality settings at 1080p (or even higher) resolutions, we still aren't seeing the full power of the GTX 1080 unleashed with this eGPU setup.

The 15W Core i7-8550U is still relatively weak compared to faster 45W gaming laptop CPUs and, of course, even more powerful desktops. The move from 2 to 4 cores under a 15W TDP limit does alleviate some of the CPU bottleneck, enough to see great performance gains, but the these sorts of ultraportable + eGPU setups are unfortunately still CPU bottlenecked.

A second major factor, the Thunderbolt 3 interface doesn't deliver quite enough bandwidth to the GPU. Even with a four-lane Thunderbolt 3 setup providing PCIe 3.0 x4, stuttering is present in some games that can't be attributed to extreme CPU loads or memory allocation issues. Granted, the number of games that stutter is less with the faster ultraportable CPU, but a game like The Witcher 3, for example, still experiences performance hitches when the CPU is only at 60% load and memory allocation is well below the limits of the system.

It is possible this is a latency issue rather than bandwidth, though either way, I'm confident it's related to Thunderbolt 3 which makes it a hard problem to resolve by simply throwing in more CPU power.

The end result is the GTX 1080 Gaming Box hooked up to a Core i7-8550U laptop provides performance most similar to the GTX 1070 Max-Q in gaming laptops, particularly in more GPU-heavy games. Across the titles we tested, the eGPU combo was 4% slower on average than a GTX 1070 Max-Q laptop with the Core i7-7700HQ inside. It's also 7% faster than a GTX 1060 laptop, which is great news for 1080p gaming.

However, the GTX 1080 eGPU paired with a current-gen 15W CPU ends up 24 percent slower on average than the GTX 1080 Max-Q, and that's not even a fully-fledged GTX 1080. So if you're thinking of buying the GTX 1080 Gaming Box or a similar eGPU to use with an ultraportable laptop, don't expect to get immediate GTX 1080 performance; instead you'll be throwing a lot of extra GPU power at what is essentially a CPU limited system.

Don't let that performance difference take away from what is still a great setup for gaming, though. Thanks to external GPUs, you can carry around a thin and light laptop for productivity on the go, then return home to a desk where you plug in just one cable to unleash very respectable gaming performance.

It's not as good as a proper gaming laptop or gaming desktop just yet, but there's a bright future for this sort of technology as some of the early issues get ironed out.