It all depends on the environment you are working in. If you are counting people passing a certain spot "high speed" might be the ability to count 10 people per second. If you are counting the particles from a nuclear reaction, the ability to count particles per nanosecond (10^9 particles per second) would "high speed".
In general, the transition between low and high speed is somewhere in the order of 1 to 10 MHZ.
Frank