If it is one-way communication it should always be email. If it's something that requires direct simultaneous input from more than 2 people, discussion on best way forward, or it's time critical always make a meeting. People always fail to answer emails in a timely manner, and when they do they only answer half the questions you asked them. So you end up going back and forth for 3 weeks before you have your answers for something that would have been resolved in a single meeting in the first week. And itgets exponentially worse with 3+ people.
Oh and the amount of people that straight up ignore email requesting information from them is staggering...fuck those people.
Speed-wise it seems to be the around the upper limit, the difference between record-pace for 10k, HM and FM is surprisingly small. They seem to mostly be pushing the duration that they can maintain these speeds.