Some of the old hands at work tell stories about working on the radar while the SR-71 was blazing through*. Nowadays the fastest we see is a pokey Dutch or Belgian F-16 running up to six or seven knots.
Pshaw.
* - Not that you'd be likely to pick up a Blackbird on a civilian radar anyways if they weren't squawking something convenient. Mostly sightings from piots below or a nice phoned-in warning from the Air Force