Webcam streamer with PyQt and Python threading, PySerial and GPIO

Question

I am developing a GUI on Raspberry Pi 3 with Qt designer and writing code in Python with PyQt. The GUI will communicate through serial port to a serial device and shall also be streaming live video from a webcam attached to Rpi3 USB.

To start with, I found a simple GUI with two push buttons (Begin and Stop) for the webcam as required by me. But the stream was little slow. So, I took examples from PyImageSearch and initiated a thread to do the webcamstreaming.

import sys
import cv2
import numpy as np
from threading import Thread
from PyQt4 import QtGui, QtCore, Qt, uic
from PyQt4.QtCore import pyqtSlot

class WebcamStream():
     def __init__(self,src=0):
         self.capture = cv2.VideoCapture(src)
         (self.ret, self.readFrame) = self.capture.read()
         self.stopped = False
         self.currentFrame=np.array([])

     def start(self):
        print "Stream_Start"
        t = Thread(target=self.update, args=())
        t.daemon = True
        t.start()
        return self

     def update(self):
        """                           
        capture frame and reverse RBG BGR and return opencv image                                      
        """
        while True:
            if self.stopped:
                return

            (self.ret, self.readFrame) = self.capture.read()
            self.currentFrame=cv2.cvtColor(self.readFrame,cv2.COLOR_BGR2RGB)
            print "Image_Updated"


    def read(self):
        print "Read_File"
        return self.readFrame

    def stop(self):
        self.stopped = True

    def captureNextFrame(self):
        self.read()
        if(self.ret==True):
            self.currentFrame=cv2.cvtColor(self.readFrame,cv2.COLOR_BGR2RGB)
            print "Frame_captured"

    def convertFrame(self):
        """     converts frame to format suitable for QtGui            """
        try:
            height,width=self.currentFrame.shape[:2]
            img=QtGui.QImage(self.currentFrame,
                             width,
                             height,
                             QtGui.QImage.Format_RGB888)
            img=QtGui.QPixmap.fromImage(img)
            self.previousFrame = self.currentFrame
            print "Frame_converted"
            return img
       except:
            return None            

class Gui(QtGui.QMainWindow):
    def __init__(self,parent=None):
         QtGui.QWidget.__init__(self,parent)
         uic.loadUi('mainWindow.ui', self)
         self.show()
         self.video = WebcamStream(src=0).start()
         #self._timer = QtCore.QTimer(self)
         #self._timer.timeout.connect(self.play)
         self.Stopp=True
         self.update()

   def play(self):
       try:
           while self.Stopp:
               #self.video.captureNextFrame()
               self.videoFrame.setPixmap(self.video.convertFrame())
               self.videoFrame.setScaledContents(True)
               cv2.waitKey(2)
       except TypeError:
           print "No frame"

   @pyqtSlot()
   def on_ButtonBegin_clicked(self):
       #self._timer.start(100)
       self.play()

   @pyqtSlot()        
   def on_ButtonStop_clicked(self):
        #self._timer.stop()
       self.Stopp=False
       self.video.stop()
       cv2.destroyAllWindows()

def main():
   app = QtGui.QApplication(sys.argv)
   ex = Gui()
   ex.show()
   sys.exit(app.exec_())

 if __name__ == '__main__':
     main()

But the stream is still sluggish. I haven't done programming with threading and even don't know whether I have done it right. But I have to do threading as I have to further use the Rpi3 GPIO and serial lines.