Qt C++ how to quickly draw in a widget window?

Question

Qt

The problem is that when drawing graphical primitives through QPainter, a lot of time is spent. I decided to make a normal raycast render. And so it turns out that while I draw the columns simply by drawing lines, it works more or less quickly, but as soon as I tried to make textures, and for this you need to draw pixel by pixel, the speed dropped to terrible.

Here is the code that slows down...

for( int y = temp; y < tempHeigh; y++ )
      painter.drawPoint(angle,y);

But this replacement works much faster.
painter.drawLine(angle,temp,angle,tempHeigh);

I doubt that it is possible to optimize somehow, the point is in the method itself... How can I get out of the situation?
You need to draw pixels, but without terrible brakes ...