问为什么在java中多线程的文件写入速度较慢？

EN

import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;

public class Main {

    public static void main(String[] args) throws InterruptedException {
        int[][] input = {new int[50000], new int[50000], new int[50000], new int[50000], new int[50000]};

        File[] outputFiles = {new File("src\\text6.txt"), new File("src\\text7.txt"), new File("src\\text8.txt"),
                new File("src\\text9.txt"), new File("src\\text10.txt")};

        long startTime = System.currentTimeMillis();
        Thread[] threads = new Thread[5];
        for (int i = 0; i < 5; i++) {
            Appender appender = new Appender(input[i], outputFiles[i]);
            threads[i] = new Thread(appender);
            threads[i].start();
            appender.run();
        }

        for (Thread thread : threads) {
            thread.join();
        }
        long stopTime = System.currentTimeMillis();

        System.out.println(stopTime - startTime);
    }
}

class Appender implements Runnable {

    private String result;
    private int[] input;
    private File outputFile;

    public Appender(int[] input, File outputFile) {
        this.result = "";
        this.input = input;
        this.outputFile = outputFile;
    }

    public void appendString() {
        for (int number : input) {
            this.result += String.valueOf(number);
        }

        try (BufferedWriter writer = new BufferedWriter(new FileWriter(this.outputFile));) {
            writer.write(this.result);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void run() {
        appendString();
    }
}