[dfox]

There are two "scan" rates: the rate at whitch keyboard matrix is scanned by whatever electronics is inside keyboard (that is probably independent of the outside interface) and the rate at which the interface is able to process input events, which for PS/2 means faster that the keyboard can produce them (as keyboard is essentially bus master on the AT/PS2 keyboard "bus") and for USB means as fast as the *HCI pools keyboard for interrupt events (old Apple's ADB works the same way and is in fact to some extent inspiration for USB <3.0 and Firewire "one big bus" high level model).

Edit: then there is third approach: simple keyboard/input devices with serial interfaces, where host provided clock is used to both clock the interface and keyboard scan logic. In essence the whole keyboard then looks like one big shift register. Keyboards that works this way includes original Macintosh, most Wyse and DEC terminals and MIT/LMI/Symbolics Lisp machines (and probably pre-sun4 Suns, sun4 and later have rs232-derived ionterface), also this is the way how controllers for Nintendo consoles before Wii work (IIRC Nintendo calls that "EXI bus") and how PlayStation 1/2 controllers work. IMHO this is to some extent where the idea behind SPI comes from.