Pro v Flash - I always thought of Flash as quick, Pro as slower, Chain of Thought enabled.

But Gemini (the app/website) lets you set the Thinking level for Flash or even Flash-Lite. And Flash with Low thinking is the default.

And that’s exactly where it starts to get murky, because this isn’t a clear gradient after all but a matrix of different qualities for different tasks. Reasoning effort levels are one factor here, but the whole speed vs. Quality topic is another.