The ACO has been trying desperately for years to reduce lap times to a level which they think is safe for the current track standards: 3:30 mins.
Each year they either reduced fuel allowance and/or added weight. And each year they failed, because of the speed of development in fuel efficiency, aero and tyres. I don't think the LMP1 will be significantly slower this year - and last years pole-setter doesn't expect it either:
http://www.motorsport.com/wec/news/neel-jani-q-a-my-le-mans-lap-record-isnt-set-in-stone-682131/