Since the yaw outputs of GNSS/INS devices are referenced by comparing GNSS acceleration with the on-board accelerometers, more movement (when GNSS is available) will result in a better yaw.
The minimal acceleration and movement for the yaw to be estimated properly is 7 m/s.
In general, the more accelerations and movement will result in a better yaw. At lower speeds the yaw might be less accurate and take longer. If the velocity is lower, a good option might be to select the GeneralMag filter profile which also uses the magnetometer to stabilize the yaw.
For more information on filter profiles, please refer to the product's specific manual, available at: https://www.xsens.com/xsens-mti-documentation