Problem wit QTimer, different time, why?

[perseo]

Hello!!
I am implementing one video reproducer and the refresh is 40 ms.
To make this i use one timer:

timer = new QTimer();
connect(timer, SIGNAL(timeout()), this, SLOT(updateLabel())); 	
timer->start(40);
 
updateLabel()
{
    XXXXXXXXXX
    Label->setPixmap(QPixmap::fromImage(qimg_l));
}

timer = new QTimer();
connect(timer, SIGNAL(timeout()), this, SLOT(updateLabel())); 	
timer->start(40);

updateLabel()
{
    XXXXXXXXXX
    Label->setPixmap(QPixmap::fromImage(qimg_l));
}

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But when i load the video the first time is slower than the second, third, ...
I use for example one refresh of 1s and the video is loaded too the first time at 1s for each frame, but the second, third, faster than the first.
Why this thing? Any idea?

Thank you