First do this (or something simmilar):
Qt Code: 
  1. void drawLines(QPainter *painter, QVarLengthArray<QLineF, 100> &lines){
  2.          painter->drawLines(lines.data(), lines.size());
  3. }
  4.  
  5. void setupArray(QVarLengthArray<QLineF, 100> &lines){
  6.          const int gridSize = 25;
  7.          qreal left = int(rect.left()) - (int(rect.left()) % gridSize);
  8.          qreal top = int(rect.top()) - (int(rect.top()) % gridSize);
  9.          for (qreal x = left; x < rect.right(); x += gridSize)
  10.             lines.append(QLineF(x, rect.top(), x, rect.bottom()));
  11.          for (qreal y = top; y < rect.bottom(); y += gridSize)
  12.             lines.append(QLineF(rect.left(), y, rect.right(), y));
  13. }
  14. void drawBackground(QPainter *painter, const QRectF &rect){
  15.          painter->setPen(QPen(Qt::darkGreen,0));         
  16.          QVarLengthArray<QLineF, 100> lines;
  17.          setupArray(lines);
  18.          drawLines(painter, lines);
  19. }
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This should allow you to measure how much time does it take to setup the array and how much to paint it.