I was initially annoyed by the Material Design specification that button text should be in all caps, and I wish that they'd explain their rationales more thoroughly. However, after a bit, here's what I think is going on:
We all know about the tendency for users to click buttons blindly; browser security warnings were notorious for a decade. All-caps text is known to be more difficult to read, and I suspect that having buttons as all-caps (in combination with strong advice to make them action descriptions rather than "OK" and "Cancel") is intended to slow users down just a tad so that they have a moment to think about the action. Once the UI is learned, it's irrelevant, and any slowdown doesn't apply.