Calculating RPM is a variation on calculating frequency. On this page I have two different ways of doing that:
One counts pulses in an interval (eg. pulses per second) and the other calculates the time interval between two consecutive pulses (ie. the period) and works out the frequency. I was able to time up to 5 MHz with one sketch and 100 KHz with the other one. Timing 2500 RPM (150 KHz) should be easy and accurate enough with the first one.