Fastest Render Codec?

ProRes is pretty fast, but an image sequence will likely be faster. There are several flavors of EXR, but that can also be really fast. AE recently-ish updated the default codec for the High Quality presets to be ProRes. But really, unless you're going to PNG or a video codec that's highly compressed, the actual encoding of frames isn't going to be the bottleneck, actually rendering the frames to RAM will be. The newly redesigned Render Queue in AE 2022 can help shed some light on where your render bottlenecks may be with its three different colors:

Bright green = frames are actively rendering
Dark green = frames have been rendered and are waiting to be written to file
Blue = written to file

I've been looking at these bars as a metric of whether a drive speed is fast enough or a bottlneck, but I think you could also glean some info about performance between codecs, as long as you purge your memory and disk cache in between renders.