I'd recommend using https://doc.qt.io/qt-5.10/qxyseries.html#replace-5
Replacing points one by one destroys a cpu. The metrics for me was just not even worth measuring - it was just an immediate hell no - forget that - I need faster updates. I like declarative QML for gui as a preference but calling replace on a single point fires way too many signals and takes way too long.
I throw my QML LineSeries to a QXYSeries and call replace in c++
In the future I'll probably just declare my series in C++. I wish I could have a better supported hybrid of c++ and QML charts but I have this working very fast using useOpenGL : true; (you can also c++ setUseOpenGL(true) )