Data rate sdk 5.1

Hi.
I would like to read data from sensors and quaternion with a rate of at least 100Hz.
I am using the MotionFit board with sdk 5.1.
In order to obtain what I want I changed DEFAULT_MPU_HZ to 100,
but what happens is that without the board detecting motion and without asking for the quaternion the rate is approximately 100 Hz, but is slows down to 40-50Hz as soon as the device detect motion (blue led lit on) and the quaternion is provided.

What would be the best way to have such a data rate?

Thank you so much