To increase the Capsense tuner refresh rate, the firmware has to make sure that the Capsense_RunTuner() API is called at the refresh rate you require (assuming you are using the synchronized tuner operation).
It is not recommended to change the sensitivity of the CapSense tuner just to increase the tuner refresh rate.
You can make sure that there is no other firmware logic that is halting the tuner operation. No delay functions, no blocking statements, no critical sections in the code will help with the tuner operation.
Tune the sensors with a simple firmware, that just scans the sensors and reports it to tuner. Once the sensors are tuned as per the requirement, tuner can be disabled and all other application firmware can be enabled.
Could you let me know what rate you are observing now? In the bottom portion of the tuner, you can observe the refresh rate in packets per second.