After switching to OpenGL the performance of plot updates became noticeably slower.
I used setOpenGl(true) and checked right after if switching to OpenGL was successful (i.e. openGl() returned true). I I used an OpenGL benchmark software to make sure there wasn't anything wrong with my machine. Everything looked fine.
QCustomPlot without setOpenGl works exceptionally well until I try to use line thicknesses above 1. I'm using Qt 5.7.