Sieve.jar employs "Sieve of Eratosthenes" algorithm and tries to use it as a benchmarking tool. You can also set the number of iterations individually (default is 100). The time it takes for your phone to calculate, is reported in milliseconds. The project has inspired by the work that can be found at: http://dada.perl.it/shootout/sieve_allsrc.html
My phone managed to finish one hundred iterations in 34 seconds.
N.B. On slow phones, it may take a considerably long time to accomplish the task. In that case, try to use a lower number as the iteration option.