Lets do some maths:
15,000 rpm = 2500 rps assuming you are getting 1 pulse per revolution.
assuming pulses are square wave then you get 1/2500 = 400usec per pulse.
So it is getting quite fast for your 8051, I suggest using hi speed cygnal microcontroller.
Another point to consider is that the 4N35 is not high speed optocoupler, check the datasheet and pick optocoupler that can handle high speed pulses.
Regards
Mahmood