Just over forty-nine years ago, in October of 1967, William J. Knight flew the North American rocket-powered X-15 at a speed of Mach 6.72 [4,520 miles per hour (7,274 km/h)], to set the still-unbroken speed record for a manned aircraft.
The speed record for an unmanned aircraft was set by NASA’s X-43A at Mach 9.6 on November 16, 2004, at an altitude of 33,223 meters over the Pacific Ocean.
The top speed for a manned aircraft taking off on its own power belongs to the SR-71 Blackbird which established in July of 1976 a still-standing speed record of (2,193.2 mph; 3,529.6 km/h), approximately Mach 3.3. There are reports of faster speeds, but not under accepted standards for records.
None of these aircraft are known to be operational today, ostensibly because the first two were experimental research aircraft that served their purposes, and because it’s been stated that the SR-71’s reconnaissance objectives can now be achieved by satellite or UAV reconnaissance at a much lower cost. Of the 32 SR-71s built, twelve were lost to mechanical or other non-combat causes. Over 800 SAMs were fired at SR-71s, and none ever hit, but the operating cost of the SR-71 exceeded $85,000 per hour, and ran $300 million a year, essentially $10 million per operating aircraft per year.
When I left the Navy, the F-14 was the top fighter [at least Navy pilots thought so], but it was retired early, despite being not only faster and able to do more than the F-18, because it required more than twice as much maintenance and because of [disputed] claims that some of the missions it could handle were no longer necessary.
The fastest commercial aircraft was, of course, the Concorde with a top speed of Mach 2.04, and, like the SR-71, it was retired from service for economic reasons, because it never made a profit, and refitting costs would have made it even less profitable.
Economics also have impacted commercial airliners. Today, on major routes, it takes longer to fly point to point than it did fifty years ago. The 1960s Boeing 707 actually cruised at faster speeds (roughly 3% faster) than does the latest B-737-800. The reason is that that newer airliner use high-bypass jet engines that are currently much more fuel-efficient at slightly slower speeds, and since fuel costs range from 26-35% of operating costs, fuel efficiency is far more highly prized than speed.
The bottom line: For both the military and for commercial air travel, economics outweigh speed.