Providing a fast and a slow "class" for flight tickets would add a lot of overhead to airline operation in many ways - way more than it could be worth it:
To have fast and slow flights, one could
Use two kinds of planes, fast and slow:
- Requires big, slow planes and fast, smaller planes, assuming fewer seats are sold in the fast, expensive class. An airline that has only one size of planes now would require a broad range of types of planes.
- Cannot handle flights that now use relatively small planes when the fraction of fast-class passengers is low, as for very few it get's just too ineffective to have a separate flight.
- The potential solution of downgrading the high-price customers to slow flights does not sound useful.
- Solving it by upgrading the slow-class passengers to a fast flight does not work in general as the fast planes needed otherwise are too small for the higher count of passengers (or would at least make planning even harder).
Use one kind of plane, used to fly fast or slow:
- Needs about twice as many planes compared to now.
- Makes seat allocation planning harder - need to estimate counts per speed class for planning, which may change depending on external events (like trade fairs at destination being consumer or business-targeted)
- Will provide not much benefit to the passenger, as the difference between the fast and the slow flight speed is limited by the physics of the planes used
- The slow speed would need to be the most fuel-effective speed, to support low prices. That is the speed of best L/D ratio - so the slow speed could not be much slower that today's standard speed, as that would cost extra fuel.
- The fast speed would be limited by the highest speed of the plane allowing acceptable fuel use, and by the power that the engines of the airplane can provide (assuming it's save to fly at that speed).
Because the fast-class speed is faster than the slow-class speed, it's above the speed of best L/D ratio. For speeds in that range, the induced drag is dominated by the form drag, and the from drag increases with the square of the speed. At some speed, looking at total fuel usage it does no longer make economic sense to go faster, and because of the square relation, I expect the possible speed difference to be "not large enough to make sense" to distinguish two completely different flight classes.
It could work, though, if we take large **airships** into account, and providing very slow, but very cheap flights, but still faster that water-based ships.