Thinking about using Unified Internal Storage on Android 6.0+ to expand your phone’s measly internal storage? Don’t even think about using it with anything other than a UHS-II SD card — even if your phone will let you use a slower card, don’t do it — your performance will be terrible.
It seems like it has to be UHS-II, for some reason (maybe it’s random r/w speeds?). I tried with even a very, very fast UHS-I, that benchmarked nearly the same, but Android wasn’t satisfied with it, giving the “This SD card is slow” warning.
The best priced one I could find out there (that you’d want to use) was a 32GB one: