This advice is similar in spirit to the advice given in a previous post, “Performance tip: Don’t starve your software of RAM.”
In many cases, performance is improved by using fewer than the maximum number of processors for Render Multiple Frames Simultaneously multiprocessing, even when you have enough RAM for all of the processors.
After Effects is a multithreaded application that can also use other forms of multiprocessing beyond just Render Multiple Frames Simultaneously multiprocessing, and it is possible for the processors to become “overscheduled” if these threads are competing for the same resources as the background processes used for rendering with Render Multiple Frames Simultaneously multiprocessing.
Therefore, the best approach is to begin by using a small number of processors for Render Multiple Frames Simultaneously multiprocessing; and then increase the number of processors used until you find the optimum number for your computer system and compositions.
For an 8-core computer system, the optimum number of processors may be 4 for some compositions, 6 for others, et cetera.
Run your own tests for your own scenarios. Remember: The reason that these settings are preferences is that the optimum values are different for different computers, compositions, and so on. There is no one “right” setting.
Keep in mind that using the Render Multiple Frames Simultaneously multiprocessing feature does not speed up the rendering of all compositions. The rendering of some compositions is memory-intensive, such as when you are working with very large background plates that are several thousands of pixels tall and wide. The rendering of some compositions is bandwidth-intensive (I/O-intensive), such as when you are working with many source files, especially if they are not served by a fast, local, dedicated disk drive. The Render Multiple Frames Simultaneously multiprocessing feature works best at improving performance when the resource that is most exercised by the composition is CPU processing power, such as when applying a processor-intensive effect like a glow or blur.
For tips on improving performance, see “Improve performance” in After Effects Help.
For details of RAM usage in After Effects CS5, see “Memory (RAM) usage in 64-bit After Effects” in After Effects Help.