It's not "The CPU is prioritized in everything". Imagine the render/export process as a conveyor belt. It start's from decoding source, then converted into internal format, then filter#1, then #2, #3, etc, then encoding to output format. You can't start next stage (say GPU-accel encoding) until previous is finished. So your process is not "CPU prioritized", it's bottlenecked, by CPU in this case. Since not all algorithms are/can be effectively pararelled, in worse case scenario you will be bonded not by CPU speed in general, but by its single core instead. Also, order of effects also may matter as to how fast it renders. Can't tell about AE, but for Premiere Pro it's definetly the case.
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