Please don't. Think about the environment. Let the industry focus on fuel efficiency rather than speed. If the ban is lifted, a lot of R&D money will go into speed.
The slower speeds waste a non-renewable resource--time. Extra time spent travelling is time that we can't get back.
That's only true if you can't make any use of the time while you're at the airport or in the air. You absolutely can, even if you're 'only' reading a physical document.
I don't think you have much to worry about. The airline industry has been focused on fuel efficiency for a long time now, and as a result, coast-to-coast flights already take significantly more time because they fly slower (for fuel efficiency). Unlike the 60s, air travel is no longer the domain of the upper-middle class and up, almost everyone does it, and the airlines profit through sheer volume, and live with a lot of competition. This is why they're always trying to weasel more profit with little fees for everything, because consumers are so price-conscious. They're not going to get that many takers if they start offering flights that are 2-3x the speed but 6x the ticket price.
Only if speed actually pays off. With fuel as expensive as it is (and if it's too cheap, just tax it some more), that won't necessarily be the case. And if it is, then getting people to fly less on economy flights is a better and economically more efficient way to reduce fuel consumption. Set an environmental goal and let the market sort out the process. Don't regulate the process itself.