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You're kind of stuck. The GPU acceleration was developed in concert with Nvidia. Just make sure you load up as much RAM as you can, and with your iMac, do not try to use multi processing for rendering or preview.
This post was mangled by voice to text on my phone.
Thanks, I'll keep that in mind. Excuse my ignorance but what is, "do not try to use multi processing for rendering or preview?" HOw do I avoid this?
Un-check the "Render Multiple Frames Simultaneously" switch. It's in your first frame grab.
Start here for information about memory and multiprocessing settings, as well as other tips for better performance:
The first thing that leaps out at me from your screenshots is that you aren't reserving any CPUs for other applications. That is bad.
BTW, After Effects will use your card for all of the GPU features except for one---GPU acceleration of the ray-traced 3D renderer. Don't think that you need an Nvidia card for anything other than that one feature.
See this page for details: