Enable GPU Acceleration in After Effects CC 2019

To enable GPU acceleration in After Effects CC 2019 (and later) look under the File menu for the Project Settings command:

GPU options are the very first available in the Project Settings window, under Video Rendering and Effects:

We recommend that you choose Mercury GPU Acceleration (Metal) on all Macs running macOS 10.13 High Sierra or macOS 10.14 Mojave, as it provides the best performance. OpenCL currently provides the best balance of performance and compatibility for those still running macOS 10.12 Sierra.