drawPolyLine very slow on large windows

Problem:

In the following code snippet, poly is a 100 point QPolygon which describes a zigzag covering most of the window.

When:

• width > 1.0 and
• p->window().width * p->window.height() > ~2200000

then the time to do the drawPolyLine is extremely long (several seconds) instead of several ms.

Other observations

The change is sudden as the window size is increased past the limit (which depends weakly on the aspect ratio of the window)

Because the window is so large, this is only seen on high DPI (4k) screents.

This is not seen when compiled in qt4, but I haven't checked in any other versions.

Turning off antialiasing does not get rid of the problem.

Changing between intel graphics and nvida graphics does not change this.

Drawing each of the line segments in poly using p->drawLine(poly[i], poly[i+1]) instead of using drawPolyLine() does remove the problem - it remains  fast (a couple ms) for any size window up to 4k.

Code Snippet:

p is a painter drawing into a QGraphicsRectItem

  p->setPen(QPen(color(), _width, style));
  p->drawPolyline(poly);