N20 motor with magnetic encoder to track revolutions

I am trying to use a N20 motor, 120rpm 6v 1:50, with a hall effect magnetic encoder in order to track the turning of a wheel on a robot car. I have managed to read in and count the signal, but it is far lower than expected and seems to be missing a lot of counts. Is this due to the micro:bit not being able to read fast input fast enough? I am currently only reading in one of the two inputs, I realize I may need to read both and do something with them for more accuracy but not sure what…

I need to be able to accurately keep track of the revolutions so I tell car to turn 90 degrees or move forward 100mm etc. Encoder is a Pololu magnetic encoder for N20 motor.

Any ideas are appreciated.