Joe has summarized both why and what effect the plenum slots have on the performance.  Here’s the link to the article that was published in the Jan-Feb 2014 issue of The Y-Block Magazine that goes into much more detail and also gives dyno testing results on those intake manifold plenum slots.

Intake Manifold Plenum Slots – Eaton Balancing  

Lorena, Texas (South of Waco)

Started out years ago for the Holley Three barrel carb with the single butterfly in the secondary.  Later, folks thought it helped with high rpm power, but in dyno testing, it causes a torque dip, and in some instances, it causes turbulence and loss of flow in the rear ports on that plane. Joe-JDC 

JDC

What is the purpose of the notch between the left and right holes in this intake?