I have an external graphics processor (eGPU) device, such as Apple External Graphics Development Kit, and I would like to use it in a Parallels virtual machine (VM). First, make sure you have connected eGPU device correctly: plug eGPU device to Mac via Thunderbolt and connect your display directly to eGPU. For details, see Apple KB HT208544. Once your eGPU device works fine in macOS, Parallels Desktop will use the available graphics resources to increase virtual machine video performance. Follow the instructions below to get it work.
NOTE: eGPU device works when Mac connected to power adapter. However if you eGPU has a built-in charging mechanism (it can also charge the computer), in this case it is not needed for the Mac to be connected to the power adapter. If your virtual machine is already running, suspend it via Actions menu > Suspend, but make sure to quit applications that use graphics, then quit Parallels Desktop. Open Finder > Applications > right-click on Parallels Desktop.app > Get Info.
Enable the option 'Prefer External GPU':. Locate your virtual machine's .pvm bundle as described in this article: https://kb.parallels.com/en/117333. Right-click on it > Show Package Contents. Locate the app file inside the package. Right-click on it > Get Info and also enable 'Prefer External GPU'. Start Parallels Desktop and move the VM window to the desired display.