Arduino Processor Loading and Baud Calculation

Audio processing?
Im not doing anything with audio frequencies - merely counting pulses to determine RPM, Km/h etc.

The ISR is only incrementing a count, which is then processed in the main loop.

RE the baud rate, if I have 20 character bytes to send each loop, then how is it I calculate which baud rate I ideally need to handle all the data, without overflowing the buffer etc..

Thanks