I remember being on a Qantas flight several years ago from Singapore to Sydney, on a 747-400. We were delayed leaving Singapore for some reason, and the captain came on the intercom after take-off to apologise for the delay. He said we would fly faster than usual to try to make up some of the time, adding that "we've got the fuel". I don't know how much faster a 744 can fly than its optimum cruising speed, but I guess even 20kts or so would make a difference over an 8-hour sector. (I can't remember how much time we did make up in the end - and anyway, I wouldn't know what was due to the higher speed cruise, and what was due to tailwinds, or ATC shortcuts around Sydney.)