Airlines today are flying their aircraft about 10% below maximum speeds, mainly to save fuel. In the (fairly distant) past, airliners were flown faster. So today it takes longer to get somewhere by air than it did in the 1960s.
Modern flight management systems are designed with airlines' concerns in mind. They usually allow crews to select flight profiles based on an arbitrary cost index that tells the FMS how economically the flight is to be conducted. Individual operators decide which cost index to enter into the FMS based on the balance between speed and fuel economy that the operator wants to see.
Speeds are slower than they used to be primarily because of fuel costs. Maintenance costs also diminish if the speeds are kept below the maximums, since there is less wear and tear on the engines. Traffic is a factor, but almost all the air traffic is in the vicinity of airports, and while routes are generally not direct, straight-line routes between airports, they aren't necessarily that far away from straight lines, either—and being "cleared direct" over large distances is far from unknown.
Anyway, the air time between DC and Tampa today is about 1.5 to 2 hours, depending on the carrier and sometimes on the aircraft or weather. Some carriers fly more slowly than others. The routes themselves are nearly a straight line. And the speeds are about 90% to 95% of maximum cruising speed. That 5-10% of speed difference saves a significant amount of fuel.
In addition, recent airliners may have lower maximum speeds than older airliners. In the old days, airliners were more designed for speed, whereas today they tend to be more designed for economy. The fastest airliner today is the Boeing 747, which can fly about 60 miles per hour faster than a Boeing 737, although neither aircraft is likely to be flown at full speed these days.
Jet airliners are most efficient when flown at fairly high altitudes and at moderate speeds—which is just where they are flown today. The on-board computers of airliners know the "sweet spots" of altitude and speed at which the aircraft consumes the lowest amount of fuel, and they also take into account other parameters such as weight, total flight distance, temperature, prevailing winds, and so on.