Hello LeoC4

                      When it comes to intermittent connection and slow speed issue happening on one particular device we would suggest you to check on that device setting or configuration since you have mentioned that it is happening on NVidia Shield so you can check with the device manufacturer to trouble shoot the issue. To improve the Wi-Fi signal and speed our suggestion will be W21, you can use it as additional satellite. When you connect different brand extender or different model extender it will just improve the signal , so if you have only one W21 device you can get another which can be setup on the surfboard app and you can use it accordingly.

Let us know if this helps.

I should have been clearer; it’s more apparent on the NVidia Shield as TV programs require better speed to stream properly.  The other four devices (2 tablets and 2 phones) show the same reduced speed off the W21.  I’m wondering if there’s a way to bypass the Shield’s WiFi antennae by connecting it directly (via cord) to an extender.  The W21 is only 20 feet away from the NVidia Shield, so it’s not a distance problem.

I don’t have a w21 to know but are you able to choose channel selection on that device? 

Make sure you’re not using any of the 5ghz dfs channels as if it detects radar, it has to switch the channels its using. it can cause drops if you’re in an area with radar/weather/etc. 

Our home is in a rural area.  The nearest radar installation is the municipal airport about 5 miles away.  I don’t experience drops, just reduced WiFi speed which causes excessive buffering.

when it happens, have you checked if that device is connected to the 2.4ghz or the 5ghz? 

Did you try changing the 5ghz channels?  Again, make sure you’re not on the DFS channels. 5 miles isn’t to far away for airport radar to reach with dfs. Airports and weather radar has pretty far reach and the router only has to be able to pick it up to be required to switch off the channels.