When the provider is really fast, people feel like they were overcharged.
This would not happen with services that people have more experience with.
Also being faster doesn't always add value to the customer, but it does help the provider, so people are unwilling to pay for it. They figure if he's fast he can do more business per day.
(Obviously this depends on the situation, but if you wait for an hour for the locksmith to arrive, him taking 1 minute or 15 doesn't really matter much to you.)