StringBuilder is faster than StringBuffer because it's not synchronized.

Here's a simple benchmark test:

public class Main {
    public static void main(String[] args) {
        int N = 77777777;
        long t;
 
        {
            StringBuffer sb = new StringBuffer();
            t = System.currentTimeMillis();
            for (int i = N; i > 0 ; i--) {
                sb.append("");
            }
            System.out.println(System.currentTimeMillis() - t);
        }
 
        {
            StringBuilder sb = new StringBuilder();
            t = System.currentTimeMillis();
            for (int i = N; i > 0 ; i--) {
                sb.append("");
            }
            System.out.println(System.currentTimeMillis() - t);
        }
    }
}

A test run gives the numbers of 2241 ms for StringBuffer vs 753 ms for StringBuilder.

• You use String when an immutable structure is appropriate; obtaining a new character sequence from a String may carry an unacceptable performance penalty, either in CPU time or memory (obtaining substrings is CPU efficient because the data is not copied, but this means a potentially much larger amount of data may remain allocated).
• You use StringBuilder when you need to create a mutable character sequence, usually to concatenate several character sequences together.
• You use StringBuffer in the same circumstances you would use StringBuilder, but when changes to the underlying string must be synchronized (because several threads are reading/modifyind the string buffer).

See an example here.