Not all rendering operations will max out all RAM and all CPUs (processor cores). Some rendering operations are I/O intensive; some are memory-intensive; some are CPU-intensive. If, for example, I/O is the bottleneck, then you won’t see either CPU or RAM usage maxed.
Also, the goal isn’t to see that 100% of the CPU and 100% of of RAM is being used. The goal is faster rendering (and other operations). Sometimes greatest performance is achieved when there’s “headroom” left over for other activity.
See this page for resources that explain in more detail: https://adobe.ly/dC0MdR
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Todd Kopriva, Adobe Systems Incorporated
After Effects quality engineering
After Effects team blog
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