I would also like to know how long the pulsin command overhead itself takes with a given frequency.

For example assume that the pulsin returned 1000 with a 20Mhz crystal would be 2000 microseconds. If It started measuring just after a change in state it would have taken say 1999 + 2000 or 3999 microseconds. Now add the overhead of the command itself and that would be the total time that the command takes.