MLT Manifold Hole Size- Too Big?

I have a friend who just built his cooler MLT and he made a CPVC manifold that has the holes drilled into it. He is getting pretty low efficiencies, like he is having a very hard time hitting 75%. Could it be because of how large the holes that are drilled in the manifold are? They are about maybe a little over 1/8" wide. They are big enough that he uses a strainer when draining the MLT because large pieces of grain husks come through near the beginning of the runnings. I dont really understand the science behind it to explain it to him, but I remember reading somewhere that the size of the holes could affect efficiencies. Can someone explain it to me?

Also, could it possibly be from his LHBS mill? He goes to Grape and Granary in Akron, OH (if anyone's been there). He uses a different store than I do, so i cant rule the crush out.

Any thoughts or input would be greatly appreciated!

My vote is the crush. I've made several different manifolds for my system, and the only thing that ever really affected the efficiency was the mill gap setting. Recently my efficiencies had slowly dropped from 85% down to 75%, so I checked the gap and it had drifted up to .039". So I cranked it down to around .032" (I originally had it set at .035"), and it jumped right back to 85%, with no differences in the recipe or process, and no husks in the recirculation. Beyond that, making sure the lauter is long, like 45-60min will also get you a few points (assuming he's fly sparging). I'd keep the holes many and small, and focus on the crush instead.

I'll have him try a different grain mill, see of that helps. I'll also let him know about draining slowly. That could very well be why I'm usually around 75 as well, I open my valve about half way. It drains in about 7 minutes.
So you basically open it to were it just trickles out?

Yeah, for a batch sparge it doesn't matter, but fly sparging you want it to really just trickle out, keeping 1 inch or so of water consistently on top of the grain bed. I tried a 30 min lauter a few batches ago just to see, and the efficiency dropped 4 points or so.