I use drawPath to draw line plots from lab experiments.
When experiments contain "noise" drawPath is very fast in drawing the curve, if the line with is 1 pixel wide. It is much slower (three hundred times) when it is 2 pixels wide. This huge difference cannot be explained only from the larger complexity of writing two pixels wide. There must be some other issue inside that endangers speed.
I enclose a file containing a sample path (pathdata.dat), and a tiny program to show the issue.

The output of this program on my PC is as follows:
drawpath time with default painter: 10 milliseconds
drawpath time with lines two pixels wide: 3028 milliseconds