Performance Explanation: code runs slower after warm up

The code below runs the exact same calculation 3 times (it does not do much: basically adding all numbers from 1 to 100m). The first 2 blocks run approximately 10 times faster than the third one. I have run this test program more than 10 times and the results show very little variance.

If anything, I would expect the third block to run faster (JIT compilation), but the typical output is:

35974537
36368455
296471550

Can somebody explain what is happening? (Just to be clear, I'm not trying to fix anything here, just trying to better understand what is going on)

Note:

• no GC is run during the program (monitored with -XX:+PrintGC)
• tested with Oracle JDK versions 1.6.0_30, 1.7.0_02 and 1.7.0_05
• also tested with the following parameters: -XX:+PrintGC -Xms1000m -Xmx1000m -XX:NewSize=900m => same result
• it the block is put in a loop instead, all runs are fast
• if the block is extracted to a method, all runs are fast (whether the method is called 3 times or in a loop makes no difference)

public static void main(String... args) {
    //three identical blocks
    {
        long start = System.nanoTime();
        CountByOne c = new CountByOne();
        int sum = 0;
        for (int i = 0; i < 100000000; i++) {
            sum += c.getNext();
        }
        if (sum != c.getSum()) throw new IllegalStateException(); //use sum
        long end = System.nanoTime();
        System.out.println((end - start));
    }
    {
        long start = System.nanoTime();
        CountByOne c = new CountByOne();
        int sum = 0;
        for (int i = 0; i < 100000000; i++) {
            sum += c.getNext();
        }
        if (sum != c.getSum()) throw new IllegalStateException(); //use sum
        long end = System.nanoTime();
        System.out.println((end - start));
    }
    {
        long start = System.nanoTime();
        CountByOne c = new CountByOne();
        int sum = 0;
        for (int i = 0; i < 100000000; i++) {
            sum += c.getNext();
        }
        if (sum != c.getSum()) throw new IllegalStateException(); //use sum
        long end = System.nanoTime();
        System.out.println((end - start));
    }
}

public static class CountByOne {

    private int i = 0;
    private int sum = 0;

    public int getSum() {
        return sum;
    }

    public int getNext() {
        i += 1;
        sum += i;
        return i;
    }
}