@scaracciolo welcome to the forum – this is a great first question!
On popularity, there are a couple of things I think – if I remember rightly to begin with QtChart wasn’t actually available for PyQt4/5. Looking at the package on PyPi (PyQtChart · PyPI) I see releases back to PyQt5.7 (2016) only. Four years is a while, but it’s not a huge amount of time when other established alternatives exist. The second reason (which follows on from this) was then that there were a lack of tutorials/documentation once it was available – because few people were using it.
I still haven’t – although I plan to start on some tutorials soon!
On performance – In principle QtChart could be faster (pure C++, no GIL, no dropping in & out of Python). But as always with these things it really depends on how much Python code is actually involved once the plot is generated. PyQtGraph actually uses internal Qt QGraphicsScene objects to build the plot canvas. The plotting itself requires your Python code + the conversion (through PyQt code) to the graphics scene. But once the scene is constructed the zooming/panning etc. of the points is all handled in Qt code.
This changes if you have any dynamic elements, but simple dots shouldn’t be going through Python. What shapes you’re drawing can have an effect – are the dots just “dots”, or are you drawing square shapes, with an outline and fill. Removing the outine/fill can speed things up.
But, other than that I think testing is the only answer – I’d actually be really interested to hear how you get on!