My tests show that you would be able to increase performance by 50-100% by eliminating the StringBuffer. If you are not getting times something in this range, you might want to run a profiler because you may have some other problems.

results:

5715 ms: 22957562 bytes written, buffered writer, no string buffer.

9236 ms: 22957562 bytes written, buffered writer, reuse stringbuffer for each line.

12359 ms: 22957562 bytes written, no buffered writer, no string buffer.

9442 ms: 22957562 bytes written, buffered writer, new stringbuffer each line.

import java.io.*;
public class Test {
	private static final int lines = 0x40000;
	private static final String data = "1234567890";
    public static void main(String args[]) throws IOException {
		System.out.println("Starting tests...");
	    PrintWriter out;
		long startTime;
		StringBuffer sb;
		File f = File.createTempFile("test", "txt");
		
		startTime = System.currentTimeMillis();
		out = new PrintWriter(new BufferedWriter(new FileWriter(f)));
		for (int i=0; i			out.print(i);
			out.print(" ");
			for (int j=0; j<8; j++){
				out.print(data);
			}
			out.println();
		}
		out.flush();
		out.close();
        System.out.println((System.currentTimeMillis() - startTime) + " ms: " + f.length() + " bytes written, buffered writer, no string buffer.");

		startTime = System.currentTimeMillis();
		out = new PrintWriter(new BufferedWriter(new FileWriter(f)));
		sb = new StringBuffer();
		for (int i=0; i			sb.setLength(0);
			sb.append(i);
			sb.append(" ");
			for (int j=0; j<8; j++){
				sb.append(data);
			}
			out.println(sb.toString());
		}
		out.flush();
		out.close();
        System.out.println((System.currentTimeMillis() - startTime) + " ms: " + f.length() + " bytes written, buffered writer, reuse stringbuffer for each line.");

		startTime = System.currentTimeMillis();
		out = new PrintWriter(new FileWriter(f));
		for (int i=0; i			out.print(i);
			out.print(" ");
			for (int j=0; j<8; j++){
				out.print(data);
			}
			out.println();
		}
		out.flush();
		out.close();
         System.out.println((System.currentTimeMillis() - startTime) + " ms: " + f.length() + " bytes written, no buffered writer, no string buffer.");

		startTime = System.currentTimeMillis();
		out = new PrintWriter(new BufferedWriter(new FileWriter(f)));
		for (int i=0; i			sb = new StringBuffer();
			sb.append(i);
			sb.append(" ");
			for (int j=0; j<8; j++){
				sb.append(data);
			}
			out.println(sb.toString());
		}
		out.flush();
		out.close();
        System.out.println((System.currentTimeMillis() - startTime) + " ms: " + f.length() + " bytes written, buffered writer, new stringbuffer each line.");

		f.delete();
    }
}