Get my new iMac to render with the GPU?

GPU Accelerated Rendering and Hardware Encoding

Premiere is largely CPU based. It will use the GPU for certain tasks and in more recent versions it can also further assist with encoding/decoding certain codecs (H264 and H265).

As long as your GPU is selected as the renderer you should be good to go.

You can also ensure that the Hardware Acceleration options are turned on in Preferences > Media

If you're monitoring your hardware usage, you can typically expect high CPU usage (up to 100% during encoding) with a little bit of GPU support. Depending on the situation that could be as low as 5-15%, or it could be as high as 50-60% when encoding to H264/5 with Hardware Encoding.

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