In my experience, a great founder should be good all three of these: 1) thinking fast, 2) thinking slow, and 3) knowing when to think fast or slow. It's easier to determine if someone isn't great at thinking fast (or doesn't know when they should think fast), for example if they don't respond quickly to an important email.

I don't think this is a binary thing. Moreover, I think the ability to meld the two and manage the balance of decisions across a growing number of stakeholders is one of the biggest challenges for founders.