From asking many dumb questions over the years ... I have been told that Airbus designed for two things:
1) Clean wings that are highly efficient to make the cruise lighter on fuel.
2) Donkeys that will do the job but not be too thirsty.
If you are a manufacturer that is moving into a crowded market (Boeing, Lockheed Mc Donnell-Douglas) then you have to have something that will attract the attention of the punters. Whilst in Europe they may have had a more favourable audience(!) the only factor they could go with was price.
Lately, with Airbus now mature in themselves and consolidation amongst the others, we see Embraer and Canadair doing exactly the same thing. Jet Blue has just ditched Airbus for Embraer.
When you can go faster for less fuel - then it will be done but if less fuel means cheaper-but-a-teeny-bit slower, then that is what you will see.
On short hauls, not much to notice. On long haul, for example, VS allow an extra 30 mins on the schedule for their A340s on JNB-LHR than their 744s. It's just less than that but the rounding up of time is neater for the timetable!
I sit to be corrected.
