I have tested this NUC and I have not found that the case causes any interference; I would suggest checking that the latest driver for that device is properly installed and using a 5 Ghz router would be better.
The wireless speed would be determined by the card and the router. Internet speed checkers are not the way to measure Wi-Fi speed, please check this article that explains the difference between Bandwidth and Throughput , It includes very good testing methods for both.
Thanks for the helpful response.
Unfortunate, I know it must be case interference to some degree. I can A/B test it with caseback on, and caseback off, and the results are significant. I get 3-4x the bandwidth with the caseback off. I can even see the results by putting the caseback on mid-test.
Perhaps I need to change the position of my internal antennas?