A look in the device manager tells me: Realtec RTLSU8191 network adapter # 2.
This means that in my six year old Medion PC of the same network adapter is installed.
CSL Computer has sent me another wireless USB adapter to the I still get worse values.
I use a AVM Fritz! WLAN USB Stick N v2 (300 Mbit / s, WPA2) I get my 60 Mbit / s.
I do not know what it is, other buyers have indeed full throughput. The customer service of CSL's class and so I forgive four star.