UPPERCASE text is harder to read because the shapes of all the uppercase letters are all rectangular and users are not used to reading text that way:
Designing Perfect Text Field: Clarity, Accessibility and User Effort
Nick Babich

Hm.. I would need to see a ton of data on this to be convinced of this one. Also, data showing font sizes when using uppercase. Of course one would have to reduce the type size if using all caps in order to compare it to capitalization of a label.

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