It needs to read more than one value. Otherwise blurring cannot happen. That's automatically more work. And also, considering the effect being physics based, even in your example, were it correct, calculating what n and m are is not trivial.
These UI elements (including the keyboard!) already blur their background, so that’s not a new cost. My 5 year old phone handles those fine. The distortion looks fancy, but since the shape of the UI elements is mostly-constant I’d expect them to be able to pre-calculate a lot of that. We’ll see when it ships!
