When looking at a datasheet for the 74HCT4051 for example all time units are in ns (nano seconds). So the limitations is probably only in the Arduino. I think that the ADC, according to the datasheet of the Atmega, in the Arduino takes between 15 and 250us to convert depending on clock. As you can see the 150 to 200 Hz should be way within limits of what could be handled, one reading takes far less time than the 5 ms you were considering.
Nice to here about the outcome, post it somewhere!
/Markus