This is strange. A pendulum’s period should be independent of mass, yet your PG shows clearly that in Havok, it is. I can’t figure out how your model of a pendulum differs from a real pendulum. I wrote up a whole thing about you should switch to shape and body instead of aggregates, but the result is the same.
Multiplying the applied force by the object’s mass does indeed make the period independent of mass. It would be interesting to calculate the factor necessary to make the model act like a real pendulum: for angles less than 15 degrees, a “seconds pendulum” is about 1 meter long.
Edit: period = 2 * PI * sqrt (length/g), where g = 9.81 and length is in meters.