Test to check performance.

public class Main {
    public static void main(String[] args) {
        int A = 77777777;
        long l;

        {
            StringBuffer sb = new StringBuffer();
            l = System.currentTimeMillis();
            for (int i = A; i --> 0 ;) {
                sb.append("");
            }
            System.out.println(System.currentTimeMillis() - l);
        }

        {
            StringBuilder sb = new StringBuilder();
            l = System.currentTimeMillis();
            for (int i = A; i --> 0 ;) {
                sb.append("");
            }
            System.out.println(System.currentTimeMillis() - l);
        }
    }
}

If we run above test, we will get 2241 ms for StringBuffer and 753 ms for StringBuilder.