Wookey,
cruise at high speed... underpowered or due to lack of lift from the wing design.... lower drag and higher cruise speed.
Interesting question and I have no idea what the answer is! Certainly we flew initially at M 0.88 but it wasn't long before .84 became the norm. I think one of the factors was fuel burn and another was tendency to catch up with other traffic in airspace that was still largely procedurally controlled (not much secondary radar in those days).