pyqtgraph plot 600 times... qimage is much slower

gui600.py

#!/usr/bin/python
# -*- coding: utf-8 -*-
"""
Update a simple plot as rapidly as possible to measure speed.
"""

## Add path to library (just for examples; you do not need this)

from pyqtgraph.Qt import QtGui, QtCore
import numpy as np
import pyqtgraph as pg
from pyqtgraph.ptime import time

app = QtGui.QApplication([])

dims = 1280, 720
nsamp = 1102  # 25ms of 44100smp/s, subsampling=1
ndat = 50
data = np.random.normal(size=(ndat, nsamp))

p = pg.plot()
p.setWindowTitle('pyqtgraph example: PlotSpeedTest')
p.setRange(QtCore.QRectF(0, -10, nsamp-1, 20), padding=0)
# p.setLabel('bottom', 'Index', units='B')

p.resize(*dims)
p.centralWidget.resize(*dims)

p.plotItem.hideAxis('left')
p.plotItem.hideAxis('bottom')


curve = p.plot()

# curve.setFillBrush((0, 0, 100, 100))
# curve.setFillLevel(0)

# lr = pg.LinearRegionItem([100, 4900])
# p.addItem(lr)

ptr = 0
lastTime = time()
fps = None


def update():
    global curve, data, ptr, p, lastTime, fps

    if ptr == 600:
        QtCore.QCoreApplication.quit()
    curve.setData(data[ptr % ndat])
    ptr += 1
    now = time()
    dt = now - lastTime
    lastTime = now
    if fps is None:
        fps = 1.0 / dt
    else:
        s = np.clip(dt * 3., 0, 1)
        fps = fps * (1 - s) + (1.0 / dt) * s
    print('%0.2f fps' % fps)
    app.processEvents()  ## force complete redraw for every plot


timer = QtCore.QTimer()
timer.timeout.connect(update)
timer.start(0)

## Start Qt event loop unless running in interactive mode.
if __name__ == '__main__':
    import sys

    if (sys.flags.interactive != 1) or not hasattr(QtCore, 'PYQT_VERSION'):
        QtGui.QApplication.instance().exec_()

qimage600.py

#!/usr/bin/python
# -*- coding: utf-8 -*-
"""
Update a simple plot as rapidly as possible to measure speed.
"""

## Add path to library (just for examples; you do not need this)

from pyqtgraph.Qt import QtGui, QtCore
import numpy as np
import pyqtgraph as pg
import pyqtgraph.exporters
from pyqtgraph.ptime import time

app = QtGui.QApplication([])

dims = 1280, 720
nsamp = 1102  # 25ms of 44100smp/s, subsampling=1
ndat = 50
data = np.random.normal(size=(ndat, nsamp))

p = pg.PlotWidget()
p.setWindowTitle('pyqtgraph example: PlotSpeedTest')
p.setRange(QtCore.QRectF(0, -10, nsamp-1, 20), padding=0)
# p.setLabel('bottom', 'Index', units='B')

p.resize(*dims)
p.centralWidget.resize(*dims)

p.plotItem.hideAxis('left')
p.plotItem.hideAxis('bottom')


curve = p.plot()

# curve.setFillBrush((0, 0, 100, 100))
# curve.setFillLevel(0)

# lr = pg.LinearRegionItem([100, 4900])
# p.addItem(lr)

ptr = 0
lastTime = time()
fps = None


def update():
    global curve, data, ptr, p, lastTime, fps
    curve.setData(data[ptr % ndat])
    ptr += 1
    now = time()
    dt = now - lastTime
    lastTime = now
    if fps is None:
        fps = 1.0 / dt
    else:
        s = np.clip(dt * 3., 0, 1)
        fps = fps * (1 - s) + (1.0 / dt) * s
    print('%0.2f fps' % fps)
    app.processEvents()  ## force complete redraw for every plot


class MyExporter(pg.exporters.ImageExporter):

    def __init__(self, item: pg.GraphicsWidget):
        super().__init__(item)

    # noinspection PyMethodOverriding
    def export(self) -> 'QtGui.QImage':
        QtGui.QApplication.processEvents()
        return super().export(toBytes=True)


e = MyExporter(p.centralWidget)
for i in range(600):
    # QApplication.processEvents() is called twice.
    # removing 1 or both calls does not affect performance.
    update()
    e.export()