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Last year, we briefly looked at external GPUs when we reviewed the Aorus GTX 1070 Gaming Box attached to a Kaby Lake-powered ultraportable. Now that Intel's 8th gen CPUs are widely available in a range of ultrabooks, with significant performance gains in hand, it's the perfect time to revisit eGPUs and determine whether the hard CPU bottleneck still exists in most games.
As a quick recap, last time we tested the GTX 1070 Gaming Box attached to a Lenovo ThinkPad X1 Carbon over Thunderbolt 3. The Gaming Box does contain a fully-fledged desktop GTX 1070 GPU, but the weak Core i5-7200U with just two cores and four threads presented a bottleneck in essentially all games we tested. The eGPU did transform the ThinkPad from a sleek ultraportable into a genuine gaming machine with decent enough performance, but the full power of the GeForce GTX 1070 wasn't unleashed by any stretch.
In fact, the GTX 1070 eGPU + ultraportable laptop combination was typically 30 to 50 percent slower than a traditional GTX 1070 gaming laptop, and could be up to 65% slower in CPU-limited titles. Stuttering was also a noticeable issue in some (but not all) games I tested with, which contributed to weak 1% low framerates.
In most games, when comparing these crucial 1% low results, the GTX 1070 eGPU was slower than a GTX 1060 gaming laptop, and often by a significant margin. Again, most of this came down to the CPU bottleneck, and also the bandwidth limitations of Thunderbolt 3. After all, Thunderbolt 3 only provides up to PCIe 3.0 x4 to the GPU minus some protocol overhead.
So in this eGPU revisit we're continuing the quest to find the ultimate portable gaming setup, and upping the ante in two regards:
First, Gigabyte kindly swapped out our GTX 1070 Gaming Box for their newer and faster Aorus GTX 1080 Gaming Box. These eGPUs come with a graphics card installed, and are supposed to retail for around $700, though with the current GPU shortage and price hikes, these boxes are rare. Hopefully, as the graphics card market returns to normal, these boxes will also become more available.
The GTX 1080 Gaming Box is pretty similar to its predecessor, so we're not going to talk much about the design and build because it's basically the same as before. Key things to note are the same compact size, and that it's still plug-and-play over Thunderbolt 3. It will also charge your laptop in the process, provided it supports USB Power Delivery.
Even though the new GTX 1080 Gaming Box uses a beefier GPU, and as a result uses a larger heatsink and fan, the box itself is no louder than the original, and doesn't run noticeably hotter either. It's impressive that Gigabyte managed to fit a full GTX 1080 and a power supply inside a device this small; it's much smaller than other eGPU enclosures on the market such as the Razer Core.
Second, and most importantly, we're testing with new ultraportables that use 8th gen Kaby Lake Refresh CPUs. With four cores, eight threads, and more than a 50% performance leap over the previous generation, there's hope that the new CPUs in these laptops will help deal with the CPU bottleneck we originally experienced.
If you want a more detailed review on the performance of the 8th gen U-series CPUs, check out our full review.
The laptops we used for eGPU testing are the HP Spectre x360 and the Razer Blade Stealth, both of which use the Core i7-8550U, the most commonly used 'high-end' 8th gen laptop CPU.
Both have four lanes of PCIe over Thunderbolt 3 -- unlike other laptops that have just two lanes -- and both have 16GB of RAM. The Spectre x360 is configured to use the default 15W CPU TDP and is a bit thermally constrained, as we explored in our review of the laptop. The Razer Blade Stealth is also a 15W laptop but manages to sustain better clock speeds, in the range of 300 to 400 MHz higher at steady state.
It's important to point out we tested everything using an external 1080p display hooked up to the GPU display outputs on the GTX 1080. Performance is noticeably reduced when sending the display signal back to the laptop's display, and we wanted to avoid any such slowdowns. We recommend anyone planning to use an external GPU to use an external display for gaming purposes, otherwise stuttering and performance losses become a factor.