not fast enough

MinPerimeterRectangle (with MillerRabiPrimality Test)

import java.math.BigInteger;
import java.util.Random;
class Solution {
     private static final Random rnd = new Random();
 public static boolean miller_rabin(BigInteger n) {
    for (int repeat = 0; repeat < 20; repeat++) {
        BigInteger a;
        do {
            a = new BigInteger(n.bitLength(), rnd);
        } while (a.equals(BigInteger.ZERO));
        if (!miller_rabin_pass(a, n)) {
            return false;
        }
    }
    return true;
}
private static boolean miller_rabin_pass(BigInteger a, BigInteger n) {
    BigInteger n_minus_one = n.subtract(BigInteger.ONE);
    BigInteger d = n_minus_one;
int s = d.getLowestSetBit();
d = d.shiftRight(s);
    BigInteger a_to_power = a.modPow(d, n);
    if (a_to_power.equals(BigInteger.ONE)) return true;
    for (int i = 0; i < s-1; i++) {
        if (a_to_power.equals(n_minus_one)) return true;
        a_to_power = a_to_power.multiply(a_to_power).mod(n);
    }
    if (a_to_power.equals(n_minus_one)) return true;
    return false;
}
    public int solution(int N) {
        // write your code in Java SE 8
        if(miller_rabin(new BigInteger(String.valueOf(N)))){
            return 2*(N+1);
        }
        int width=0;
        int minPer=Integer.MAX_VALUE;
        for(int length=1;length<=N;length++){
           if(N%length==0){
               width=N/length;
               minPer=Math.min(minPer,2*(length+width));               
           }
        }
        return minPer;
    }
    
}