Converting starch to sugars requires that first the starch is gelatinized. The larger the grain particles, the longer it takes to get the mash water to the starch to gelatinize. Once gelatinized the enzymes can convert. Then you need to get the sugars that are trapped in the grain particles back out into the wort.
When you start making the grain particles smaller you will eventually run into problems getting the wort out of the mash. If you make the grains particles too small they will plug up the filter.
To increase your efficiency (brewhouse?) you can mash longer so there is more time to gelatinize the starch, convert it, and extract the sugar. The problem with that is that unless you add heat, before all the conversion is done the mash has cooled so conversion stops. Adding heat has a tendency to denature the enzymes needed for conversion so you can lose that way too. Milling finer can help and adding something for a filter that doesn't plug so easily will help. People add rice hulls (cheap! add a handful) or...they use a different filter like a bag. That is where BIAB works well as you can mill the grains very fine and still not get a stuck mash as the bag has a huge filter area.
It may have been me that mentioned the 30 minute mash as I have used iodine to see if there is unconverted starch in the grain particles and I have used my refractometer to verify that the amount of sugar in the wort has quit going up.
If you have iodine, try an experiment with water and flour (not going to mill finer than that) Heat a small amount of water to 155F. and stir in a bit of flour. Remove a tiny bit of the flour and add a drop of iodine, starting at 5 minutes and continuing the experiment at 5 minute intervals until the iodine no longer changes color. Give a report back on what you find.