If your mash temp was above 170, you'll get minimal efficiency because the enzymes get denatured at those temperatures fairly quickly. Also, if you didn't buffer your mash pH, you may have extracted tannins at those temperatures.
I forgot to pre-heat my mashtun when I brewed yesterday and overcompensated the strike water temperature, so I was quite high (in the mid-160s) to begin with. Although I managed to get down to my target of 152 within 10 minutes and extended the mash by 15 minutes, I still ended up missing my target gravity slightly. I also forgot to add 5.2 at the beginning of the mash, but thankfully I remembered and added it after a few minutes.