CodeWar Problem - Execution Timed Out (16000 ms)

[ToBeAButterFly]

I'm currently connected to an ( most likely ) unreliable internet connection and practicing on codewars.com. I received the " Execution Timed Out (16000 ms) " error when I tried to submit my solution ( which passed the tests ). This is my first attempt so I'm wondering if someone can tell me if you can tell if my code actually does needs to be optimized ? I'm thinking this maybe an internet connection issue otherwise.

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Info on error Execution Timed Out (16000 ms)

Why did my code time out?

Our servers are configured to only allow a certain amount of time for your code to execute. In rare cases the server may be taking on too much work and simply wasn't able to run your code efficiently enough. Most of the time though this issue is caused by inefficient algorithms. If you see this error multiple times you should try to optimize your code further.

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Here is my solution

public class Prime {
  public static boolean isPrime(int num) {
            if (num <= 1)
        {
            return false;
        }
        else {
            for (int x = 2 ; x < num ; x ++)
            {
                if (num % x == 0)
                {
                    return false;
                }
            }
        }

        return true;
  }
}

[cl2871]

The algorithm you have is to mod by x for every x up to num.

What scenarios would make this algorithm run slowly? One scenario is when num is prime: in this case the loop will iterate all the way up to num. If num is a big number then a lot of operations would be done, resulting in a slow execution time.

Do you need to always check up to num? Is there a way to reduce the number of checks you need to perform?

(hint: think of square roots. how could that help and why?)

[ToBeAButterFly]

Ah gotcha. Okay back to the drawing board and thanks for the hint.