shortest time output between each pin? what are you trying to ask?
if you are asking how long it takes for the microcontroller to complete a given instruction, you will have to look at the Atmel's 8-bit AVR instruction set. Some instructions may take just one cycle, some take more.
1 cycle = 1/16000000 Hz = 0.0000000625 seconds = 62.5 nanoseconds
You can set the output for the PWM pins. I'm confused as well as to what you're asking but this might answer your question?