Clipping in a widget's paintEvent slows down the program

I have a problem with clipping in qt. I have a widget in which I paint a big array of rectangles. As I only change a few rectangles from time to time I want to paint only a little part of the widget new (only these rectangles), I want to clip the paint area to these parts. The isNew() function is true, if the rectangle has got a new color since the last time painting. void Environment::paintEvent(QPaintEvent *event) {     QPainter painter (this);     Tile t;       //paint the matrix     for(int i=0; i<size; ++             t = matrix[i+j*yizeY];             if(t.isNew()){                 painter.setClipRegion(QRegion(t.getRect()));                 painter.setBrush(t.getColor());                 painter.drawRect(t.getRect());                 t.used();         }     } } What am I doing wrong? My program is running even slower, when I use the setClipRegion(…) function.