Did you try to show vodeo capture only from the slow one? I think it might be usefull to test if the second camera slow only when image from both cameras is shown or it's always slow.
Another suggection is to try to swap devices. It may be some kind of hardware problem.
The last suggestion is to try to use QGraphicsView as soon as it might be just quicker than labels. However I don't think that the problem you described is in labels.