Okay, I'll bite... My first post here. And ain't it a doozy.
Early in my all-grain homebrewing "career" (about 6 years ago), in the interest of minimizing the length of brew day, which had been taking more than 5 hours and I only wanted maybe 4-4.5 hours, I ran a multitude of batches varying the mash time for each batch to determine the minimum mash time necessary to achieve adequate efficiency.
Well, the first thing I learned from my experiments was, if you mash for anything over like 20 minutes, efficiency isn't affected significantly at all. At the time I was seeing consistent efficiency of around 77% on average, regardless of mash time. Not a bad result, and not the one I had expected either...
What I did notice is that with mashes of less than 30-35 minutes, the fermentation would sometimes quit early, with an attenuation of only mid-60s in a lot of cases. This did NOT happen with every batch, and it did not happen with every yeast strain. I figure the odds of getting a good batch with good attenuation in the 70s or low 80s with a very short mash time are roughly 50/50. Seems difficult to predict from batch to batch.
The next thing I found was that if I mashed for at least 40 minutes, then every batch turned out spot on perfect with respect to both efficiency and attenuation. So, I have been mashing for 40 minutes pretty much ever since, except for those cases where I got lazy or distracted or purposely wanted a bone dry saison or something like that. But for about 90% of recipes that specify 60-90 minute mashes, I mash for only 40 minutes, and I get results that I cannot distinguish from what was intended.
Try it for yourself and see. Happy mashing. :)