Can anyone explain why the following recursive method is faster than the iterative one (Both are doing it string concatenation) ? Isn't the iterative approach suppose to beat up the recursive one ? plus each recursive call adds a new layer on top of the stack which can be very space inefficient.

    private static void string_concat(StringBuilder sb, int count){
        if(count >= 9999) return;
        string_concat(sb.append(count), count+1);
    }
    public static void main(String [] arg){

        long s = System.currentTimeMillis();
        StringBuilder sb = new StringBuilder();
        for(int i = 0; i < 9999; i++){
            sb.append(i);
        }
        System.out.println(System.currentTimeMillis()-s);
        s = System.currentTimeMillis();
        string_concat(new StringBuilder(),0);
        System.out.println(System.currentTimeMillis()-s);

    }

I ran the program multiple time, and the recursive one always ends up 3-4 times faster than the iterative one. What could be the main reason there that is causing the iterative one slower ?