Hey, you primo-function is actually very slow. There are algorithms to calculate prima-numbers in a much more performant way. You could read about here for example: Sieve of Atkin
I made some small improvements, but there is still room for more:

1. boolean primo(int p) { 
     // if p is dividable by 2, return false
     if (p%2==0 ) return false;
   
     // begin with 3, test every second number, and stop when m*m is higher than p
     for (int m=3 ; m*m < p ; m+=2) {
       if (p % m == 0) {
         return false;
       }
     }
     return true;
   }
   

Thanks a lot Benja. Your function really improves performance.