A pattern of lights displayed on a linear set of LEDs (today) or nixie tubes (yesterday, on ancient mainframes). The pattern is shifted left every N times the operating system goes through its main loop. A swiftly moving pattern indicates that the system is mostly idle; the speedometer slows down as the system becomes overloaded. The speedometer on Sun Microsystems hardware bounces back and forth like the eyes on one of the Cylons from the wretched Battlestar Galactica TV series.
Historical note: One computer, the GE 600 (later Honeywell 6000) actually had an analog speedometer on the front panel, calibrated in instructions executed per second.