The "high update frequency" servos (which I'm just hearing about) is still vague to me. The below might be what is being talked about. The update frequency a standard servo seems to be limited by requiring at least 6ms between updates. Digital servos may have this minimum dead time reduced, but there still has to be some dead time between commands.
but 120Hz is about the limit for most standard servos. This is a result of the NE544 specs (the original servo amp) that requires 6ms of "off" time between pulses.