Checking about that JSONObject#write(Writer) is more faster than Writer#write(JSONObject#toString()).

result.txt

NumberOfProperties,toString[ms/1000exec],write[ms/1000exec]
100,107,81
200,157,126
300,237,195
400,329,278
500,410,339
600,490,417
700,571,482
800,672,562
900,754,628
1000,838,710

SpeedCheck.java

import java.io.BufferedWriter;
import java.io.FilterOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.Writer;
import java.util.Arrays;

import org.json.JSONException;
import org.json.JSONObject;

public class SpeedCheck {
	private static final int EXEC_TIME = 1000;
	private static final int TEST_TIME = 20;

	public static void main(String[] args) {
		try {
			new SpeedCheck().test();
		} catch (IOException e) {
			e.printStackTrace();
		} catch (JSONException e) {
			e.printStackTrace();
		}
	}

	private void test() throws IOException, JSONException {
		long[] toStringTime = new long[TEST_TIME];
		long[] writeTime = new long[TEST_TIME];

		JSONObject object = createJSONObject(1);
		testToString(object);
		testWrite(object);

		for (int j = 100; j <= 1000; j += 100) {
			object = createJSONObject(j);
			for (int i = 0; i < TEST_TIME; ++i) {
				System.gc();
				long time = System.currentTimeMillis();
				testToString(object);
				toStringTime[i] = System.currentTimeMillis() - time;

				System.gc();
				time = System.currentTimeMillis();
				testWrite(object);
				writeTime[i] = System.currentTimeMillis() - time;
			}
			System.out.printf("%d,%d,%d%n", j, median(toStringTime), median(writeTime));
		}
	}

	private long median(long[] arr) {
		Arrays.sort(arr);
		return (arr[arr.length / 2] + arr[(arr.length - 1) / 2]) / 2;
	}

	private void testToString(JSONObject object) throws IOException,
			JSONException {
		Writer writer = createWriter();
		try {
			for (int i = 0; i < EXEC_TIME; ++i) {
				writer.write(object.toString());
			}
		} finally {
			writer.close();
		}
	}

	private void testWrite(JSONObject object) throws IOException, JSONException {
		Writer writer = createWriter();
		try {
			for (int i = 0; i < EXEC_TIME; ++i) {
				object.write(writer);
			}
		} finally {
			writer.close();
		}
	}

	private Writer createWriter() throws IOException {
		return new BufferedWriter(new OutputStreamWriter(
				new FilterOutputStream(null){
					@Override
					public void write(int i) throws IOException {}
					@Override
					public void close() {}
				}));
//		return new BufferedWriter(new FileWriter(File.createTempFile("SpeedCheck", ".tmp")));
	}

	private static JSONObject createJSONObject(int num) throws JSONException {
		JSONObject obj = new JSONObject();
		for (int i = 0; i < num; ++i) {
			obj.append(Integer.toString(i), Integer.toString(i));
		}
		return obj;
	}
}