After Effects is a great tool with an incredibly diverse user base. While it's most commonly used as a motion graphics tool it's also probably the most widely used compositor as well. For those of us in the Arch-Viz industry it's often serving in that capacity but something many people might not know is that incorrectly setting your Multiprocessing too high might be causing you significantly slower renders.
For those of us that are 3D artists this might come as a big surprise. After all - who's ever heard of a 3D rendering that got slower with more CPU's??? Compositing is different because of the extremely high data demands. In AE for every CPU participating in the rendering it needs to pull another copy of frames. So if you're using a reasonably fast workstation with say 24 cores, AE will try to pull 24 frames and depending on your comp each of those "frames" to render might be sourcing another couple dozen frames that it's comping together... And then your hard drive explodes trying to pull all those frames.
While I'd love to load up my machine with ioFX SSD cards to "fix" the problem that's not feasible for me and probably not for many artists given the cost. The free solution to maximize your AE compositing performance is specific to your individual workstation running After Effects. How fast your drives are, how many cores you have and how much RAM is available all play a part in the "right" config. In my case I'm running a dual Xeon with 24GB of RAM and dual SATA drives. It's intended to be a great 3D workstation and it is - but I had to limit it for After Effects to get the most performance out of it.
In the graph below you'll see a performance curve where on the left is 1 CPU and render time falls / rises as the Multiprocessing CPU count is upped to 12. For my machine going past 4 CPU's is a waste of RAM and generally slower (50% slower in the multi channel EXR test) when run all the way at 12 CPU's. Performance won't be the same for everyone due to differences in network, cpu, drives, etc; use this opportunity to run some tests yourself - I think you'll be surprised by your results.
Tests performed on Dell T7500 Dual Xeon, 24GB RAM, Dual SATA, nVidia Quadro 4000, ProEXR highest performance version (1.8). All footage streamed from local D drive and comp output written to C drive.