Apple External Graphics Development Kit: Will eGPU Really Elevate Your Mac?

The idea of an eGPU is fantastic. Attaching a desktop class GPU to a laptop and supercharging the GPU performance what isn’t to like. However its not all sunshine and rainbows. After delving deep into the world of eGPUs we found a few things that aren’t advertised when you purchase.

When a graphics card is plugged into a desktop system, it is inserted into the PCI-e slot on the motherboard and connected directly to the PCI-e controller. That gives it direct access to the processor with dedicated PCI-e lanes. The GPU has as much bandwidth as it needs with the only limitation being the card itself.

However, that is not the case with an eGPU. Firstly you plug in your eGPU via your Thunderbolt 3 port which is a quarter of the speed of a PCI-e port. The Thunderbolt 3 port then sends its data via the board chipset which also handles all your other devices. The data also shares bandwidth on the Direct Media Interface which connects the northbridge and the southbridge.

So what does all that mean? That means you’re getting one hell of a bottleneck…

The percentage in lost performance compared to PCI-e with no additional devices.

THUNDERBOLT # PCIE SPEED EXTERNAL DISPLAY 1080P INTERNAL DISPLAY
Thunderbolt 3 32Gbps-TB3 ~20% performance drop ~20% 1060 | ~28% 1070 | ~30% 1080 & 1080Ti
Thunderbolt 2 16Gbps-TB2 ~20% performance drop ~24% GTX 1060 | ~40% GTX1080 & TITAN X
Thunderbolt 1 10Gbps-TB1 ~20% performance drop ~50% performance drop
PCI Express 126Gbps-PCIe
Credit to eGPU.io for the results

Your performance will already be down due to the above factors but if you decide to plug in any other devices such as HDD enclosures you will see further drops in speed. If you are wondering why the performance drop is worse when running the internal display, that would be due to the additional signal display data being sent back to the internal display taking up more bandwidth and slowing it down.

So is an eGPU really worth the investment, well that depends. If you love the portability of a laptop but need that additional performance then, by all means, get an eGPU. The Apple development kit is being shipped with an RX 580 which is similar performance to the 1060. Performance difference should sit around 20% compared to standard desktop performance. There is certainly a place for eGPUs but until we see a large bump in port speed or we get a dedicated propriety connection which connects the GPU direct to the PCI-e bus its not an absolute solution.

If you have any further questions, please do feel free to get in touch with our team.


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