I've been having a hard time finding any reputable research or studies discussing whether established design guidelines (i.e. Material Design, iOS guidelines) are easier for people to use than custom app design or not.
I know the theoretical reasons why design guidelines are so strongly encouraged (people know what to expect from apps, consistency, better performance, etc.) but I want to know if there are any studies that explicitly user test this somehow.
Similarly, I know that Google has done a lot of research and testing in the development of Material, but I can't find anything on what that research and testing actually entailed. Is there any article or paper somewhere that describes their research?