100% CPU WHEN RENDERING

AE doesn't really use GPU...

Or rather, its effects are designed on a per-effect basis. Many only go to 8 bit, some aren't very compatible with time effects, while a few use GPU... but I feel it only uses GPU in some base effects, like for shape layers animated with expressions, or color grading without any other effects. 

So, in short, this is expected behavior. It will use all the CPU it can, and the render process is duplicated so each core can work on a different frame. (I do not consider this proper multithreading, as the threads do not communicate or work on the same output frame. It's a trick AE uses to hide that its rendering capability and its entire backend is primarily single-threaded)

as a 30 year old software, it's not well known to be optimized or performant on newer systems. I upgraded from an nvidia 680 with an intel xeon 3440, to an nvidia 1070 with an intel 6700k. I didn't really notice a difference... and AE has gotten chunkier and less performant per operation, so neither meets AE's minimum spec any more.

So, I reiterate: that's just how AE works, and unless you do mograph, I actually suggest turning GPU off, as it can cause instability for some people, and with some GPU archetectures.

Any chance you have Multi-Frame Rendering enabled? If so you should be able to reserve some CPU bandwidth through the Memory & Performance tab under preferences

Hi there!

Thank you for reaching out. I'm Sameera, an engineer on After Effects. Do you mind sharing the project that you are trying to render? That will help me narrow down more on the root cause. If not, please check to see if your project contains any features / effects that are not supported on the GPU yet. You can view the GPU-accelerated effects by going to Project Settings > Video Rendering and Effects. Thanks!