QSqlDatabase and execution query time: increases each time!

[Wing_Zero]

Hi at all,
I'm sorry to be annoying but I'm new to qt and sometimes there is something that i really can't understand

Specifically, I've a QSQLITE DB and when i try to do an insert or update query (no problem with select query) just like this:

QString query = "UPDATE MovieDB SET ...etc...";
    QSqlQuery queryResult= QSqlQuery (db);
    queryResult.exec (query);
    std::cerr << qPrintable(queryResult.lastError().text());
    emit doneUp();

QString query = "UPDATE MovieDB SET ...etc...";
    QSqlQuery queryResult= QSqlQuery (db);
    queryResult.exec (query);
    std::cerr << qPrintable(queryResult.lastError().text());
    emit doneUp();

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each time i invoke this function the time take to execute the query increase by one step.
For example if the first time I take 0.5 seconds. the second time 1.0 seconds. then 1.5 seconds, and so on...
I'm really in panic because I think there is nothing I can do to fix this...
The lag problem, unluckily, it is only a query execution fault: in fact commenting that line [ queryResult.exec (query); ] the lag disapper.
I don't know how to solve that.

I appreciate every help, and thanks is advance.

As usually sorry for my bad english

Gianluigi