Thanks John Dong. The Wireless Adapter we are talking about is a Netis WF2190 AC1200. Wireless Chipset is from Realtek. Adapter has 2 Streams for Transceive and 2 for Receive. The Realtek Software shipped with this Adapter reporting me a (theoretical) speed of 867Mbit/s Up and Down. This whould be a dream. 😉
So if i understand you right it could be a Problem with the Wireless Adapter / Antenna itself. Unfotunatelly i dont have other Adapters for testing with Windows 7.
If there are any other ideas for my gap issue, please give me a feedback. If someone has a good experience with a USB AC Adapter working fine with Ruckus (R500), dont be shy and write it down. Thanks.