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Problem solved
Please look below if you have the same issue.
The solution is to turn off “Group Key Rotation” in your Wifi Router.
I discovered the message
wpa_supplicant[323]: wlan0: WPA: Group rekeying completed
in journalctl
And after this message occurs everything went slow. It does not play a role if you use an internal or external Wifi device.
Iperf3 seems to give false readings.
This is no config issue with my router. Turning Group Key Rotation off ist not very funny. This is standard in WPA2 and not other device has a problem with it.
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What happend?
If I start an *.mkv file from my NAS the movie starts stuttering and sound is muted.
This behaviour does not stop at any time.
Media Info
http://paste.osmc.io/eficogeweq.tex
Log
I switched debug on and restarted the Pi 3 and reproduces the problem. The problem starts at ~15:03:49 until the end of the log
http://paste.osmc.io/lofabudume.coffee
CPU is at 100%
edit:
I guess the interesting path is this:
15:04:50 285.137512 T:1487922160 NOTICE: CDVDPlayerAudio::OutputPacket skipping a packets of duration 32
15:04:50 285.169586 T:1956283312 NOTICE: Previous line repeats 7 times.
I used Notepad++ to count the occurences of “OutputPacket skipping a packets” and the result was 4457.