SensorMUX establishes a direct channel of communication between
NXT and your sensor.
To establish that channel, this block needs to send commands to
SensorMUX.
Also, after selecting channel NXT needs to configure the sensor
based on it's type (such as Light/Touch, etc).
All that takes time, so avoid changing channels frequently in
loops.
Your program should change the channel, and then in a Loop Block
(or a Wait Block) read the sensor, once your sensing criteria is
met, move on to next sensor.
For example see the program below.