I wrote a quite test program running a timer 120 times a second to see what sort of variation in time period I got.

The app was the basic view template application with the timer scheduled using:

[NSTimer scheduledTimerWithTimeInterval:1.0/120.0 target:self selector:@selector(myTimer userInfo:nil repeats:YES];

The timer behaves nicely when there is no user interaction going on. The timer function get's called approximately every 0.008333 seconds. Things get a lot more variable when you have the user touching the screen.

Here's my result running over a 30 second period:

No touches

0.006008,3
0.006832,3
0.007655,1763
0.008479,27
0.009302,3
0.010126,0
0.010950,0
0.011773,0
0.012597,0
0.013420,0
0.014244,1

Touching, tapping swiping the screen etc

0.002097,5
0.003344,25
0.004591,29
0.005838,247
0.007085,1258
0.008332,1726
0.009579,252
0.010826,27
0.012073,24
0.013320,5
0.014567,1

Multi-touch enabled with touching tapping and swiping the screen

0.002818,9
0.003929,14
0.005039,23
0.006150,202
0.007261,2684
0.008371,433
0.009482,189
0.010593,21
0.011704,14
0.012814,9
0.013925,1