Why QCustomPlot is too slow in ploting large data?

I want to plotting some large chunk of data (3k) which is coming every 100ms. I tried QCustomPlot and Qwt with exact 3k points and i got really good performances in plotting with Qwt and really bad performances with QCustomPlot. And i think i behave wrongly with QCustomPlot, i used this code for plotting in QCustomPlot (This example is from QCustomPlot plot-examples which i edited function setupQuadraticDemo):

void  MainWindow::setupQuadraticDemo(QCustomPlot *customPlot)
{
 demoName = "Quadratic Demo";
 customPlot->addGraph();
 customPlot->setNotAntialiasedElements(QCP::AntialiasedElement::aeAll);

 customPlot->xAxis->setRange(0, 1000);
 customPlot->yAxis->setRange(0, 1000);

 customPlot->xAxis->setLabel("x");
 customPlot->yAxis->setLabel("y");

 connect(&dataTimer, &QTimer::timeout, this, [customPlot]
 {
  constexpr auto length = 3000;
  QVector<double> x(length), y(length);

  std::srand(std::time(nullptr));

  for (int i = 0; i < length; ++i)
  {
   x[i] = std::rand() % 1000;
   y[i] = std::rand() % 1000;
  }

  customPlot->graph(0)->setData(x, y, true);

  customPlot->replot();
 });

 dataTimer.start(100);
}

And this code for Qwt. Is i do wrong with QCustomPlot ? Why it's too slow in plotting ?

I guess that the root of the problem is the code itself. You are updating points in the wrong way. You have to delete the below line from your code

std::srand(std::time(nullptr));

This line will force the rand()'s seed to be fixed for a deterministic time(if I want to be precise your seed value is fixed for 1 second), so whether the data itself is updated or not you cannot see any changes because the replot is going to plot the same points for that duration(1 sec).