Code for determining beats per min when timing a wooden clock
If the accuracy you require is about 1%, that should be doable with any
Arduino. Much higher accuracies would be hard to achieve because most
Arduinos are clocked off a ceramic resonator, and these have a frequency
tolerance of about 0.5%.

Given that your signal is very slow, the simplest approach may be to
poll the pin connected to the sensor and record the time (using
micros()) of the low →&nbsp;high transition. Then you can compute the
beat frequency as the inverse of the time between consecutive
transitions.

There is, however, the possibility that the sensor is not exactly in
the middle of the swing. Then, for better accuracy, I would measure the
time taken by two beats, i.e. a full swing. This way the pssible
asymmetry will not affect the accuracy.