My reader says it can do update rates up to 35 Hz, depending on protocol and how many codes you're reading. This is certainly fast enough for speedo. When I am reading all codes, the tach readout lags quite a bit. I'll have to try limiting codes to just RPM and Speed and see how it does.
It depends on how many values you are requesting at the time. If only one (mph) then it will have a faster refresh then if you are asking for two (mph / rpm), and so on. With a quality / well engineered OBD2 hardware / software, the car is the slowest link in the chain as far as updating frequency.