There is no such thing. This is simply not how AE works. Many operations simply are single-threaded because they require linear progression.
Working in comps won't scale. But, you can improve your rendering performance (using AE CC 2014 or earlier) by setting AE to use 4 to 8 cores, and as much RAM as you can spare. I use 8 out of 32 cores, 6GB allocated to each thread (yes, it's a big workstation), and the rendering scales very well. Previews, too. There's a wait of about 10 seconds before all the threads launch, then the speed kicks in. Why they removed this feature without having a viable replacement is a mystery to me. But, the interface changes in 2013 were so annoying that I'm still using 2012 (the first "CC").