Question about speed: Weird Behavior

I'm doing some speed tests, and I noticed something very strange.

My test code:

        Connection con = DriverManager.getConnection("jdbc:postgresql://localhost:5432/tcc","postgres","postgres");
        PreparedStatement pst;
        ResultSet rs;
        Timer timer = new Timer();
       
        timer.start();
        pst = con.prepareStatement("select count(*) from Curso");
        rs = pst.executeQuery();
        rs.next();
        timer.stop();
       
        timer.start();
        pst = con.prepareStatement("select * from Curso");
        rs = pst.executeQuery();
        rs.next();
        timer.stop();

At every stop, I print the milliseconds occurred between the begin and the end of the operation. The results were basically

1st  operation: 0.05 miliseconds
2nd operation: 0.1 miliseconds

That's ok, the strange thing is: When I comment the first operation and re-execute the test, the execution time of the 2nd operations increases by 50% (going to 0.15 miliseconds)

If i inverse the order of operations, the speed is changed again, the 2nd (now 1st) get the 50% percent increase and the count(*) get even faster going to 0.005 miliseconds.

At first glance, I thought that it could be the statement pre-compiled but the two statements are different so it doesn't apply (i think). Using the Wireshark tool i could notice that the package sizes get bigger automatically (going from 8258 to 16450 and finally to 49218). From my readings, the Java Sockets have some configurations about buffer size that maybe is doing this.

Even when I close the connection and open a new one, the new queries still get faster.

--

Anyway, that speed increases is definitely a good thing, but why this happens it's a mystery for me. The problem is that my benchmark tests are influenced with this, cause the order of the tests can influence the outcome of the total time.

Does someone know what's is this? And how could I disable this (if possible)?

Thanks in advance,

Israel