82 seconds to extract one row in the db, java.lang.OutOfMemoryError: GC overhead limit exceeded, with large database

I added 100 000 rows to a the table in database (localhost) and since then I get this error:

java.lang.OutOfMemoryError: GC overhead limit exceeded

I resolved the problem by typing in consol:

javaw -XX:-UseConcMarkSweepGC

And the consol the output is (see code below for context) :

2015-08-02T02:57:22.779+0200|Info: 5
2015-08-02T02:57:22.779+0200|Info: end, time taken: 82755

It takes 82 seconds to extract one row in the database(see code at the end). It was working fine when I had less rows so I'm wondering:

• Why would it take so much time to extract 1 row ? JPA can't possibly extract every row in an object ? Or does it ? Just wow.
• Is there a way around this ? I mean extracting a single row in 80 seconds is borderline slow.
• Do I really have to type the command -XX:-UseConcMarkSweepGC? What does it do ? From the doc :

Use concurrent mark-sweep collection for the old generation. (Introduced in 1.4.1)

Here is how my code is:

@EJB
private ThreadLookUpInterface ts;

@Schedule(hour = "*", minute = "*/1", second = "0", persistent = false)
@Override
public void makeTopThreadList() {
    System.out.println("" + ts.getThread(5).getIdthread());
}

my service ejb like this:

@Stateless
public class ThreadLookUpService implements ThreadLookUpInterface {

    @PersistenceContext(unitName = "my-pu")
    private EntityManager em;

    private static final String FIND_THREAD_BY_ID = "SELECT t FROM Thethread t WHERE t.idthread=:id";

    @Override
    public Thethread getThread(int threadId) {
        Query query = em.createQuery(FIND_THREAD_BY_ID);
        query.setParameter("id", threadId);
        try {
            Thethread thread = (Thethread) query.getSingleResult();
            return thread;
        } catch (NoResultException e) {
            return null;
        } catch (Exception e) {
            throw new DAOException(e);
        }
    }
}