To add to what Bruce is saying…
If you are starving your cores of RAM, they start up when you start a render (such as a RAM preview), starve to death, shut down, and then try again the next time that you start a render.
That’s why you should really read this: “Performance tip: DOn’t starve your software of RAM”.
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Todd Kopriva, Adobe Systems Incorporated
putting the ‘T’ back in ‘RTFM’ : After Effects Help on the Web
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