To measure how long a single code line or chunk of code need toexecute, you have several method
1: Scope method
you Set/Clear an I/O before and after a code chunk, then you measure it with a scope

2: MPLAB StopWatch
You set a breakpoint at the end of the code chunk you want to measure, You set the PC at the start of the code chunk, then you click on Run at PC (refer to MPLAB help file)

3: use a PIC timer
Same concept as StopWatch but you need to send the results to a LCD or via serial communication. The following link explain it better

instruction execution time
http://www.picbasic.co.uk/forum/showthread.php?t=365

i'll try to figure out what the heck happen here... would be nice if i had those shift register