We have actually talked directly with AMD about eGPUs. These must be used with an external monitor to accelerate graphics-apps like X-Plane on an iMac or laptop, so that the eGPU can feed the monitor directly. You may also need to set the external monitor to the 'main' monitor in system prefs too, if X-Plane keeps getting the wrong GPU.
If you have set your eGPU up in this manner and still have performance issues, please send a bug report with your log.txt here.
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