0672055a changed the Material widget to
always use Paths for representing the outline.
These paths are later used for clipping and drawing a shadow.
This changed introduced a performance regression, see:
https://github.com/flutter/flutter/issues/14403

We did not expect a path that is a rounded rectangle to be less
performant than a rounded rectangle, as Skia should be able to tell the
path is just a rounded rectangle.
Until we find a solution for this regression, we keep using RRect when
we can represent the shape with it.