Is the aux part big fan a design flaw or I missed something?

This is probably because you did not uncheck :

“Slow printing for better cooling” on Filament Settings
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And

“Slowdown on bridge” on Speed Settings :
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This is all fine to keep those boxes checked, but you are loosing a lot of speed and you may have some differents quality trouble because of speed variation wich is never good on a fast print, you can look this forum thread as an example : [SOLVED] Bad quality when printing PETG - #201 by Thrawn