Hello,

I have a simple test program that plots 10 curves with 288 points each:

.h
Qt Code: 
  1. class MyPlot : public QwtPlot
  2. {
  3. public:
  4. 	void drawCanvas(QPainter* painter) override
  5. 	{
  6. 		const auto start = std::chrono::system_clock::now();
  7. 		QwtPlot::drawCanvas(painter);
  8. 		const auto end = std::chrono::system_clock::now();
  9. 		const auto dt = std::chrono::duration_cast<std::chrono::milliseconds>(end - start);
  10. 		qDebug().nospace() << "dt = " << dt.count() << "ms";
  11. 	}
  12. };
  13.  
  14.  
  15. class QwtPlotClient : public QDialog
  16. {
  17. 	Q_OBJECT
  18.  
  19. public:
  20. 	QwtPlotClient(QWidget* parent = nullptr);
  21.  
  22. private:
  23. 	Ui::QwtPlotClientClass m_ui;
  24. 	MyPlot* m_plot = nullptr;
  25.  
  26. };
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.cpp
Qt Code: 
  1. namespace
  2. {
  3. 	std::vector<QVector<QPointF>> createPlotData(int curveCount = 10, int pointsPerCurve = 288)
  4. 	{
  5. 		std::vector<QVector<QPointF>> result;
  6. 		std::default_random_engine engine(std::random_device{}());
  7. 		std::uniform_real_distribution<double> distribution(-100.0, 100.0);
  8. 		for (int curveIndex = 0; curveIndex < curveCount; curveIndex++)
  9. 		{
  10. 			QVector<QPointF> curvePoints;
  11. 			double x = 0.0;
  12. 			for (int point = 0; point < pointsPerCurve; point++)
  13. 			{
  14. 				curvePoints << QPointF(x, distribution(engine));
  15. 				x += 2.0;
  16. 			}
  17. 			result.push_back(curvePoints);
  18. 		}
  19. 		return result;
  20. 	}
  21. }
  22.  
  23. QwtPlotClient::QwtPlotClient(QWidget* parent)
  24. 	: QDialog(parent),
  25. 	m_plot(new MyPlot)
  26. {
  27. 	m_ui.setupUi(this);
  28. 	m_ui.plotFrame->layout()->addWidget(m_plot);
  29.  
  30. 	const auto curvePointsList = createPlotData();
  31. 	for (const auto& curvePoints : curvePointsList)
  32. 	{
  33. 		auto* curve = new QwtPlotCurve();
  34. 		curve->setSamples(curvePoints);
  35. 		curve->attach(m_plot);
  36. 	}
  37. }
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In my main.cpp I define if the program uses high dpi settings or not:
Qt Code: 
  1. int main(int argc, char *argv[])
  2. {
  3. 	// Comment the following 4 lines for NOT using high dpi settings
  4. 	QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
  5. 	QGuiApplication::setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy::Round);
  6. 	QCoreApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);
  7. 	QCoreApplication::setAttribute(Qt::AA_ShareOpenGLContexts);
  8.  
  9. 	QApplication a(argc, argv);
  10. 	QwtPlotClient w;
  11. 	w.show();
  12. 	return a.exec();
  13. }
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As you can see I measure the time that the qwt plot needs for drawing the canvas (see MyPlot::drawCanvas). The results are (on my computer, you probably will have different absolute values):

High dpi settings off: ~25ms (for Debug) / ~10ms (for Release)
High dpi settings on: ~800ms ms (for Debug) / ~570ms (for Release)

I tested this with Qt 5.14.0 and Qwt 6.2.1. (I also tested with Qwt 6.2.0 and the numbers for high dpi settings on are even a bit larger.)

Is there anything I can do? Or do you plan to "fix" this in the near future?