The tick frequency is set by configTICK_RATE_HZ in FreeRTOSConfig.h. If you set it faster than 1KHz then the portTICK_RATE_MS constant will not work, so the demo applications will also not work. The demo applications are separate to the kernel though. It will not prevent the kernel working, but will be extremely inefficient as you will spend a large amount of time in the tick interrupt and pointlessly switching tasks. You are better configuring a separate timer to generate an interrupt at that frequency if you need it.