Nice work @pijyoi !

now the question I have is should we consider removing the experimental flag to use openGL with PlotCurveItems?

Leave it as experimental. The OpenGL path is not at parity, feature-wise, with the regular path. "connect" is not supported, for example.

It's not clear to me that it even provides a speed benefit.

It's not clear to me that it even provides a speed benefit.

Definitely faster in the case of pen width greater than 1, which is a major performance regression we have seen significant demand to try and address.

On a particular Windows machine, using examples\PlotSpeedTest.py modified to disable vsync:

Looks like useOpenGL alone would be good enough for the thick pen widths?

Oh huh, I clearly have something crossed up, I thought enableExperimental had to be set for the openGL to take effect for PlotCurveItem.

Regardless, This PR LGTM, merging. Thank you so much for tracking this down @pijyoi

I also noticed that the axis fonts were no longer crisp with useOpenGL enabled.

EDIT:
From https://doc.qt.io/qt-5/qopenglpaintdevice.html
"Antialiasing in the OpenGL paint engine is done using multisampling. Most hardware require significantly more memory to do multisampling and the resulting quality is not on par with the quality of the software paint engine. The OpenGL paint engine's strength lies in its performance, not its visual rendering quality."