WiFi vs Wired Performance - huge difference

I also have strange wifi issues. I also used an external usb wifi antenna and deactivated the internal antenna but the results are the same. I just get 9-15/Mbit/s (to my local NAS and to the Internet). The distance between router and Pi does not play a role ( I moved the Pi to the router → same result)
I also have video stuttering issues which could be connected to my wifi issues.

Ping to NAS:

— 192.168.0.35 ping statistics —
27 packets transmitted, 27 packets received, 0% packet loss
round-trip min/avg/max = 1.585/4.837/7.999 ms

<img src=“/uploads/default/original/2X/6/660e890ec504a68a33b25fda259d9b86bfce637c.png” width="668"height=“419”>

OS:
root@osmcwohn:~# uname -a
Linux osmcwohn 4.4.27-5-osmc #1 SMP PREEMPT Tue Nov 22 02:23:12 UTC 2016 armv7l GNU/Linux