100 points in just a millisecond:

1) find prime numbers from 1 to N. 2) find maximum power of prime number. 3) multiply each prime powers.

Hint: You dont need to calculate nonprime numbers because you always have enough primes to make them up

int result = 1;
for (int i = 1; i <= n; i++) {
  if (isPrime(i)) {
    int r1 = 1;
    while (r1 * i <= n)
      r1 *= i;
    result *= r1;
  }
}
return result;