What is the effect of brown out detection on frequency?
Mostly all microcontrollers have built-in Brown-out detection (BOD) circuit, which monitors supply voltage level during operation. BOD circuit is nothing more than the comparator, which compares supply voltage to a fixed trigger level. If the supply voltage goes below that level the controller will be hold in reset.  

See Brown Out Info for details.

To answer the question: No, a BOD circuit has no influence on frequency or timing.