Rurouni
Veteran
Basically they are trying to save fuel as much as possible because they can only generate very little energy from MGU-K and the engine is detuned to probably below the maximum efficiency (as in km/l) the engine could do. They are also limiting the fuel flow to save fuel. So they run at relatively low speed and the driver need to drive in a manner to save fuel, thus lots of lift and coast, which in effect making it hard to get the tires and brakes into the correct operating window.I can't find reference to it online now, but I thought I heard a post race interview with Button also pointing out that when they were running at the speeds and the manner they were it was also difficult keeping the tires and brakes in in the correct operating window, thus further impacting performance.
Near the end, they got enough fuel to burn and they increase the fuel flow, thus boost the McLaren performance to get that 1:33. So basically in a detuned state, the engine still got more performance to give, but limited in performance because of the fuel saving McLaren had to do to reach the finish line.