I'm no expert, but I'll give it a shot -
You've said that the fastest aircraft are the 777 and 747, followed by the A330/A340. You might also notice that these are also the longest range. For a regional jet, MD-80, or 737 flying 1000 miles or less, speed is not imperative because most of the scheduled time is cusioning for traffic and delays. On intercontinental flights, the relatively small difference does make a difference.
Eg: 565 miles per hour is the published cruise speed of the 747-400, and I'll estimate 500 miles per hour for a smaller regional aircraft.
* ATL-GPT (351 miles) - @500mph = 0:37, @565mph = 0:42
Once you add the time to takeoff and climb and waiting in traffic, that's almost no difference.
* ORD-HKG (7793 miles) - @500mph = 15:35, @565mph = 13:48
Time to takeoff and climb and waiting in traffic is roughly the same whether you're going 500 miles or 5000, so here the near 2 hour difference makes a big difference. So you can see why, for the larger and longer range aircraft, it is worthwhile to put forth the extra research and development time and money in squeezing out the highest cruise speed possible without harming the fuel economy.