Well, I used to buy my grains pre-crushed. I got 75% efficiency
Then I switched, and crushed with a Corona mill. I got 75% efficiency
Then I got a JSP roller mill. I got 75% efficiency but the milling time was much shorter
Then I started doing a mash out (fly sparging). Efficiency jumped to 85% - consistently
Perhaps I was lucky with the quality of the pre-crushed grains.