[![enter image description here][1]][1]<BR>([Source][2])

In a turbojet, slowing down the exhaust defeats the purpose of a *jet* engine.

And if you mean to use all the core's energy to run the fan in a turbofan, the outer bypass flow will diverge into the now low-pressure zone, causing drag, and might even cause flow reversal, i.e., air will be ingested from the back—causing a surge.

Also—at max power—a fan in a turbofan *already* runs at its most efficient speed, any faster and the gains will turn into losses.


  [1]: https://i.sstatic.net/QPYw4.png
  [2]: https://commons.wikimedia.org/wiki/File:Turbofan_operation.svg