Startup founders need to do two things: (1) figure out what to do, (2) do it. (I suppose that applies to anyone, but I'm writing about founders). My beef with "move fast" is that I don't naturally associate it with figuring out what to do. To me, "move" is physical, outward, observable. Shipping features and talking to people... but not thinking. If I spent a day thinking, trying to better understand a problem, I wouldn't view myself as having "moved fast" that day. And that's a real shame, at least if "good founders move fast" is true.
Perhaps people who advocate for moving fast don't intend to exclude thinking. Perhaps "be proactive" would be a more technically accurate way to get the idea across. Using my definitions, moving fast is a strict subset of being proactive.
The problem with the phrase "be proactive" is that it doesn't have the same emotional punch as "move fast," and in many situations, the narrow definition of "move fast" is in fact what you should be doing. So I prefer to keep the phrasing, but I reject the statement "good founders move fast all the time."
To be clear, I'm not sure if anyone actually accepts that statement. It is plausibly just a figment from the part of my brain that generates FOMO. Sam Altman at least seems to agree with what I'm saying:
Almost everyone I’ve ever met would be well-served by spending more time thinking about what to focus on. It is much more important to work on the right thing than it is to work many hours. Most people waste most of their time on stuff that doesn’t matter.
Once you have figured out what to do, be unstoppable about getting your small handful of priorities accomplished quickly. I have yet to meet a slow-moving person who is very successful.
How do you know when you've figured out what to focus on? Hammock-driven development is some A++ related content. The talk is given in the context of software development, but it's not about software—in fact it's quite interesting to read from the context of startups.
I'd like to add my own small rule of thumb: great solutions are often surprisingly simple. If you can explain your idea to someone and finish by saying "that's it," there's a decent chance you're on the right track.
 For example, I think a lot of techniques for startup "validation" are best viewed within the framework this talk presents. Techniques that give you more input to think about are good, but often, the decision to pursue an idea or pivot ultimately comes down to well-informed intuition.
 After re-reading the transcript, I see Rich covered that too: "if I know that I've got an answer and the answer is small, that's one of the most telling attributes that [it] is probably good."