Unusable rtsp video playback (More than 90% of frames are dropped)

When playing back RTSP streams on imx6 platfprm via QML MediaPlayer element, the playback is extremely bad:

• Playback needs nearly 5 seconds to start
• Playback drops about 90% of frames
• Warning message on console output: Warning: "A lot of buffers are being dropped."

When using a native gstreamer pipeline like this on the cmdline, playback starts after about 1 second and is close to perfect:

gst-launch rtspsrc location=rtsp://axis-cam1/axis-media/media.amp ! decodebin2 ! mfw_v4lsink sync=false

I can replicate the Qt5 bad playback somewhat partly when using *sync=true' on the videosink element (playback start is delayed several seconds and the warning appears, but frame drop is still somewhat better than in Qt).

On a PC platform (Linux amd64) the effect is not so strong: Playback is also delayed several seconds and there are often hickups in the playback, but frame drop is rather something like 10% and not 90%.