How to increment log frequency in micro SD using sparkfun imu razor 9 dof

Hi I have to log in a micro sd acceleration and velocity from an mpu9250 at least at 200Hz, the producer of the board ensures that the board is capable to reach 200Hz but his firmware can’t log over 100Hz, anyone has tried to encrease log speed of a sparkfun razor imu 9 dof board? I tried to modify the open-source firmware but i think that the bottle neck is the data format, in fact now I store strings data in a txt file. I don’t mind the format of the file. I have to post-process data with Matlab so I can convert log file at anytime after the acquisition
I’m interest in log the time stamp (or time from past sample) and 6 integers 16 bit

The board has this hardware configuration:
Atsamd21 microprocessor
Mpu9250 triaxial gyroscope and accelerometer
MicroSD reader