Using appsink&qlabel to display rtsp worked not fluently

Here is my code to get rtsp:

pipeline = "rtspsrc location={} latency=0 ! rtph265depay ! h265parse ! avdec_h265 ! videoconvert ! appsink".format(cam_adrs)
cap = cv2.VideoCapture(pipeline, cv2.CAP_GSTREAMER)

Then, I put every frame on a QLabel and use a QTimer to display:

ret, frame =
if ret:
    # transform format
    img = QImage(frame, frame.shape[1],  frame.shape[0], QImage.Format_RGB888)
    pixmap = QPixmap.fromImage(img)

It displayed with no delay, but not fluently, almost like stop-motion animation.
I played the rtsp by gst(using autovideosink). It’s fluent.
I‘ve tried 15fps、20、25,but not useful.

So,What are the possible causes and solutions?