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, and the fast speed could be limited by the fastest safe speed of the plane allowing acceptable fuel use. Hard to estimate meaningful numbers, but I expect the **possible speed difference** to be "not large enough to make sense".

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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.