If you were thinking about installing an RTX 4090 into your shiny new external GPU box, you might want to reconsider. JarrodsTech on YouTube recently released a new video showcasing what the RTX 4090 can do with an external Thunderbolt GPU dock connected to a laptop. The results were incredibly bad. In his testing, the dock/laptop combo crippled the 4090's gaming performance, with most games barely being able to use 50% of its power, at best.
For testing, Jarrod used two different laptops: an Asus Zenbook 14 OLED featuring a i7-1260P processor, and a beefier Aorus 17X gaming laptop equipped with a higher-performing Core i9-12900HX. Jarrod also put together a 7950X desktop test rig as a baseline, to see how much performance the 4090 retains in the external GPU dock configuration. This means we have quite a bit of a mismatch in CPU performance between the laptop and desktop PC CPUs, which is important to bear in mind because the RTX 4090 can bottleneck CPUs in some game titles.
Unfortunately, Jarrod neglected to share details on the external GPU dock he is using, only noting that it's a Thunderbolt 3 dock -- which is good considering most GPU docks today are using Thunderbolt 3. But, Jarrod did switch to an external Corsair 850W power supply for the tests, since the built-in 650W unit didn't have enough 8-pin power connectors for the 4090's quadruple 8-pin power adapter.
In testing, Jarrod found that all titles tested could barely use 50% of the RTX 4090's GPU horsepower, with most being much worse. God of War was one of the best games tested, representing a 35% reduction in performance at 4K on the i7-1260P Zenbook, compared to the desktop 4090 configuration.
But in a worst-case scenario, the external 4090 was up to 82% slower than the desktop counterpart in Watch Dogs Legion at 4K.
To make matters worse, the performance difference between both laptops was also drastically different in most titles. A good example of this is in Assassin's Creed Valhalla, where the 12900HX gaming laptop was up to twice as slow as the ultrabook with the slower i7-1260P processor.
This game was definitely the worst offender, but many other titles exhibited similar behavior, including Red Dead Redemption 2, Call of Duty Warzone, Watch Dogs Legion, and a couple more.
The behavior is very bizarre, especially when the 12900HX is a significantly faster part. Jarrod suspects the Resizable BAR feature might have something to do with it, but he isn't certain. His 3DMark PCI Express feature tests measured 2.33 GB/s of bandwidth for the TB3 enclosure, and at least some of that shortfall could be due to the eGPU design. However, even at full utilization, the TB3 interface provides far less bandwidth than the 26.85 GB/s he measured with the desktop PC.
In either case, it seems like pairing an RTX 4090 with a modern-day Thunderbolt 3 GPU dock and a lower-power laptop CPU will result in very poor utilization of the 4090. Thunderbolt 3 GPU docks simply aren't designed to drive a 4090 well, even at 4K resolutions.
We will have to wait for much higher-performing Thunderbolt 4 or (perhaps even Thunderbolt 5) GPU docks before cards like the RTX 4090 make sense as external gaming options. But with the capabilities of high-end GPUs always increasing, thus putting more demand on both the CPU and the external interface, it's unclear if the highest-end GPUs will ever reach similar performance to traditional desktop setups. So for the foreseeable future, it's clear that if you want the best gaming performance possible, you should opt for one of the best gaming PCs or best gaming laptops rather than an external GPU.