A cost index is only one part of it..there are other factors to consider as well.
A high speed descent can avoid a hold, a runway closure, or make a schedule that includes connections on other flights.
The cost of time IMHO is immensly undervalued, simply because accountants dont have any idea what it costs a company in goodwill or reputation for a late arrival/departures (all they use is effectively their best guess)..simply because they only deal in absolutes. i.e. direct operating costs
