swift first 100 prime numbers (slow)

import Foundation

func checkNum(num: Int) -> Bool {

    for i in 2...num - 1 {
        if num % i == 0 {
            return false;
        }
    }
    return true;
};

var numArray = [2,3]; //known small prime numbers

var counter: Int = 4;
while numArray.count != 100 {
    if checkNum(num: counter) {
        numArray.append(counter);
    }
    counter += 1;
}

for i in 0..<numArray.count {
    if i % 10 == 0 && i != 0 {
        print("");
    }
    print(numArray[i], " ", terminator: "");

}