system clock is too slow

hi, I was using CC3220SF-LAUNCHXL to do some project. I use I2C to read the data, and I set the fequency to 400kHz, but I only got 325k Hz for reality. Then I using GPIO to do I2C, but I only got 130k Hz for reality. So my question is how to set the internal clock frequency to get a faster GPIO?   THANKS!