Using XCode and instruments to improve iPhone app performance-Collection of common programming errors

I’m not sure instruments is your best bet here. Instruments samples your code, meaning (amongst other things) that it affects timing. If you want to know how long a loop takes then you should put something in the code to calculate and either retain or display the amount of time each loop is taking.

An actual runtime can be had by:

// activity starts
NSDate* startTime = [NSDate date];

// activity ends
NSLog(@"time elapsed this loop is %f", fabs([startTime timeIntervalSinceNow]));

Getting an average is easy if you have a time calculated per loop since you can sum the times, count the loops and divide before you display the average.

Anything in your loop affects the timing, of course, but this affects in a minimal way.