Proxy layers render slower than original. Video layers in general are extremely slow.

Pre-rendered, thank you, that was the term I was looking for.

"On your specific issue, how have you used or re-introduced your pre-render into the render chain? ANd have you disabled the composition and all of its layers that were used to render these pre-renders after you introduced the pre-render into the render chain? "

Exactly.

Say for instance I have composition with a background with some elements, some video footage on top of that, and on top of video footage some other elements. I'd render background and front elements as 2 pre-rendered videos, turn off those layers and replace them with the prerendered versions, compound all 3 (background, main video, top) as final export. On the M1 this is excruciatingly slow. As a test, I put one of those pre-rendered layers in a single comp and export it, a single video layer, just to see where the timesink was in the composition. To my surprise this video only rendered at 3 fps. Much slower than the original composition it replaced. On my old windows workstation the prerendered layers would render fine. So this really feels like an Apple Silicon issue.