I believe the southbound Highlander sleeper was timed at "90" mph in order to get a path between the morning southbound Pendolinos into Euston. (In fact the timing load for 90 mph was used, but extra half-minutes performance allowance {} added here and there to take account of the class 92's maximum speed being 87 mph = 140 kph). It wasn't (then) possible to path it at 80 mph in the up morning peak. With fewer services running now, and different stopping patterns, that may have changed.