J2EE - PostgreSQL - JDBC - Slow query

I am experiencing a strange behaviour with a PostgreSQL database and JDBC driver (in a J2EE application).

To keep it simple, let's say I have two tables :

SHIP_MESSAGE(id, datetime, latitude, longitude)
SUBMARINE_MESSAGE(id, datetime, latitude, longitude, immersion)

Each one of them contains messages that have been emited at a given datetime. Now I have to replay these informations on a website.

I am using AJAX to request the infos around every 300-500ms. The request is something like : select id, datetime, latitude, longitude from SHIP_MESSAGE where date >= '2013-02-11 18:00:00' order by date limit 1;

If I execute it once (to get the message from the first table), it takes around 150 ms, which is fine. But if I execute it twice (to get the messages from both tables), each query takes between 800 and 2000 ms !

So this works fine :

MessageDAO dao = new MessageDAO(); // Data access object, used to execute my query

Date d1 = new Date();
ShipMessage message = dao.getShipMessageAtDate(date);
Date d2 = new Date();

System.out.println( d2.getTime() - d1.getTime() ); // Around 150 ms

And this doesn't :

MessageDAO dao = new MessageDAO();

Date d1 = new Date();
NavigationMessage message = dao.getShipMessageAtDate(date);
Date d2 = new Date();
SubmarineMessage m2 = dao.getSubmarinMessageAtDate(date);
Date d3 = new Date();

System.out.println( d2.getTime() - d1.getTime() ); // Between 800 and 2000 ms
System.out.println( d3.getTime() - d2.getTime() ); // Between 800 and 2000 ms

I'm using a singleton pattern to get my connection. If I create two Connection objects, it works fine (~ 150 ms), but I don't want to do that because I won't be able to open enough connections when there will be too much clients.

Any idea ?

I tried to use a join to have only one request containing the data of both messages, but it is also too long (1-3 sec), which is weird because if I execute it directly in the terminal it is fast.

Thanks !

EDIT :

Here are the code for the MessageDAO :

public class MessageDAO extends DAO {

    public MessageDAO() { super(); }
    public MessageDAO(Connection connection) { super(connection); }

    public NavSensorsMessage getShipMessageDate(Date date) throws SQLException {

        String sql = "select * from BOAT_MESSAGE where date >= ? order by date limit 1;";

        PreparedStatement ps = _connection.prepareStatement(sql);
        ps.setTimestamp(1, new java.sql.Timestamp(date.getTime()));

        ResultSet result = ps.executeQuery();
        result.next();

        Date datetime = result.getDate("datetime");
        float latitude = result.getFloat("latitude");
        float longitude = result.getFloat("longitude");

        return new ShipMessage(datetime, latitude, longitude);
    }
}

Here is the class DAO :

public abstract class DAO {


    // -------------------------------------------------------------------------
    protected Connection _connection;
    // -------------------------------------------------------------------------



    // -------------------------------------------------------------------------
    // Constructors :
    public DAO() { _connection = StaticPostgreSQLConnection.getInstance(); }
    public DAO(Connection connection) { _connection = connection; }
    // -------------------------------------------------------------------------
}

And here is the StaticPostgreSQLConnection :

public class StaticPostgreSQLConnection {


    // -------------------------------------------------------------------------
    private static final String _driverName = "org.postgresql.Driver";
    private static final String _url = "jdbc:postgresql://localhost:5432/telesciences";
    private static final String _user = "mylogin";
    private static final String _password = "mypassword";
    private static Connection _connection;
    // -------------------------------------------------------------------------



    // -------------------------------------------------------------------------
    public static Connection getInstance() {

        if (_connection == null) {
            try {
                Class.forName(_driverName);
                _connection = DriverManager.getConnection(_url, _user, _password);
            }
            catch (ClassNotFoundException | SQLException e) { e.printStackTrace(System.err); }
        }
        return _connection;
    }
    // -------------------------------------------------------------------------



    // -------------------------------------------------------------------------
    public static void close() {

        try {
            _connection.close();
            _connection = null;
        }
        catch (SQLException e) { e.printStackTrace(System.err); }
    }
    // -------------------------------------------------------------------------



    // -------------------------------------------------------------------------
    public static void begin() throws SQLException { getInstance().createStatement().execute("BEGIN;"); }
    public static void commit() throws SQLException { getInstance().createStatement().execute("COMMIT;"); }
    public static void rollback() throws SQLException { getInstance().createStatement().execute("ROLLBACK;"); }
    // -------------------------------------------------------------------------
}

EDIT 2 :

Here is a piece of postgres log :

2014-02-12 11:02:02 CET LOG:  durée : 0.122 ms, analyse <unnamed> : select * from NAVIGATION_MESSAGE where date >= $1 order by date limit 1
2014-02-12 11:02:02 CET LOG:  durée : 0.143 ms, lien <unnamed> : select * from NAVIGATION_MESSAGE where date >= $1 order by date limit 1
2014-02-12 11:02:02 CET DÉTAIL:  paramètres : $1 = '2011-07-02 01:08:05.16'
2014-02-12 11:02:02 CET LOG:  exécute <unnamed>: select * from NAVIGATION_MESSAGE where date >= $1 order by date limit 1
2014-02-12 11:02:02 CET DÉTAIL:  paramètres : $1 = '2011-07-02 01:08:05.16'
2014-02-12 11:02:02 CET LOG:  durée : 157.295 ms

2014-02-12 11:02:02 CET LOG:  durée : 0.114 ms, analyse <unnamed> : select * from NAVSENSORS_MESSAGE where date >= $1 order by date limit 1
2014-02-12 11:02:02 CET LOG:  durée : 0.161 ms, lien <unnamed> : select * from NAVSENSORS_MESSAGE where date >= $1 order by date limit 1
2014-02-12 11:02:02 CET DÉTAIL:  paramètres : $1 = '2011-07-02 01:08:04.88'
2014-02-12 11:02:02 CET LOG:  exécute <unnamed>: select * from NAVSENSORS_MESSAGE where date >= $1 order by date limit 1
2014-02-12 11:02:02 CET DÉTAIL:  paramètres : $1 = '2011-07-02 01:08:04.88'
2014-02-12 11:02:02 CET LOG:  durée : 157.598 ms

Every request takes only 150 ms, so the problem is not postgres itself. I guess it comes from the way I get my connections.