eGPU support in Mac/macOS I'm wondering whether an external GPU can be (almost) as efficient as the same internal GPU? I need to have a fast computer even when I have a lot of windows open on many virtual desktops and when I run resource intensive application on two 4K displays.

Is macOS able to use an eGPU for its own needs (system animations, etc.)?
Yes. Using an external GPU with your Mac (with support for eGPU) can give your computer a performance boost.

Refer to the Apple support article, Use an external graphics processor with your Mac.


  An eGPU can give your Mac additional graphics performance for professional apps, 3D gaming, VR content creation, and more.
  
  eGPUs are supported by any Thunderbolt 3-equipped Mac running macOS High Sierra 10.13.4 or later.


For a list of Thunderbolt 3 equipped Macs and to check if your Mac support eGPU, refer to the Apple support article, Adapters for the Thunderbolt 3 (USB-C) or USB-C port on your Mac or iPad Pro

These Macs have Thunderbolt 3 (USB-C) ports:


  
  iMac Pro
  iMac models from 2017
  Mac mini (2018)
  MacBook Pro models from 2016 or later
  MacBook Air (Retina, 13-inch, 2018)