Hibernate: query performance drop after a few repetitions

I am developing a Farm Management system. One of the basic queries is to list the animals by type that are alive (% with%). Below is the method that performs this query.

public List<T> findByInFarm(T model, boolean inFarm) throws DAOException {

    List<T> animalList = null;
    EntityManager em = JpaHelper.getEntityManager();
    Query query = em
            .createQuery("SELECT a FROM " + model.getClass().getSimpleName() + " a WHERE a.inFarm = :inFarm");
    query.setParameter("inFarm", inFarm);
    try {
        long start = System.currentTimeMillis();
        animalList = (List<T>) query.getResultList();
        long end = System.currentTimeMillis();
        System.out.println("findByInFarm() demorou " + (end - start) + " millis");
    } catch (Exception e) {
        throw new DAOException(e.getCause().toString());
    } finally {
        if (em.isOpen()) {
            em.close();
        }
    }
    return animalList;
}

The query returns the expected result, but after running the same query a few times, performance drops a lot. I inserted a inFarm = true before System.currentTimeMillis() and one later to measure query performance and rode the test below.

public void test() {
    while (true) {
        try {
            List<Cow> cowList = new Cow().findByInFarm(true);
            new Thread().sleep(2000);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Below is the result obtained after some repetitions:

• findByInFarm () took 1017 millis
• findByInFarm () took 615 millis
• findByInFarm () took 606 millis
• findByInFarm () took 591 millis
• findByInFarm () took 566 millis
• findByInFarm () took 561 millis
• findByInFarm () took 586 millis
• findByInFarm () took 588 millis
• findByInFarm () took 546 millis
• findByInFarm () took 3036 millis
• findByInFarm () took 4876 millis
• findByInFarm () took 4959 millis
• findByInFarm () took 4904 millis
• findByInFarm () took 4924 millis
• findByInFarm () took 4936 millis

The first queries are very fast but after the tenth repetition time increases a lot. Could anyone give a hint where the possible causes might be?

Note: I'm using Hibernate 5.2, Postgresql 9.4 and Java 8.