Via WSJ:

Why a Six-Hour Flight Now Takes Seven

.... Across the airline industry, carriers have been adding minutes to "block times" — the scheduled durations — baking delays into trips so that late flights officially arrive "on-time" and operations run better because flights pull into gates more often on schedule.

.... lengthening scheduled arrival times boosts on-time rankings charted by the Department of Transportation: Those numbers can have a real effect on public perception. And in some cases, block times have grown simply because airlines have been making so many schedule changes as they have reduced capacity over the past two years. Flights that took off without a wait can now end up stuck waiting behind a line of jets because departure times have been changed.

On the positive side, it increases the chances a traveler will catch a connecting flight. On the negative side, it means fewer planes in the air, thus more expensive tickets, and significantly more time spent on travel for frequent fliers.