Java - Synchronized methods cause a program to slow down massively

Question

I am trying to learn about threads and synchronization. I made this test program:

public class Test {
    static List<Thread> al = new ArrayList<>();

    public static void main(String[] args) throws IOException, InterruptedException {
        long startTime = System.currentTimeMillis();

        al.add(new Thread(() -> fib1(47)));
        al.add(new Thread(() -> fib2(47)));

        for (Thread t : al)
            t.start();
        for (Thread t: al)
            t.join();

        long totalTime = System.currentTimeMillis() - startTime;
        System.out.println(totalTime);
    }

    public static synchronized int fib1(int x) {
        return x <= 2 ? 1 : fib1(x-2) + fib1(x-1);
    }

    public static synchronized int fib2(int x) {
        return x <= 2 ? 1 : fib2(x-2) + fib2(x-1);
    }
}

This program takes about 273 seconds, but if I remove both of it synchronized, it will work in 7 seconds. What makes this huge difference?

EDIT: I know that I'm using a terribly slow Fibonacci number calculation algorithm. And I also know that threads do not share resources, and therefore methods do not need to be synchronized. However, this is just a test program in which I try to understand how it works synchronized, and I chose a slow algorithm so that I can measure the time spent in milliseconds.

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public static int fib1(int x) {
   synchronized(LOCK1) {
        return x <= 2 ? 1 : fib1(x-2) + fib1(x-1);
    }
}

public static int fib2(int x) {
   synchronized(LOCK2) {
        return x <= 2 ? 1 : fib2(x-2) + fib2(x-1);
    }
}

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