I think one of the culprits is that much of the playtest was done with the Caves of Chaos dungeon, which feature almost no interesting tactical situations (e.g. archers on balconies, fire pits into which to push foes) or monsters with interesting abilities. It's all pretty much just "charge in and make attack rolls". And that kind of stuff is fairly low effort, particularly if you play it without a grid, so it moves fast, and you can quickly move on to the next fight. In an environment like that, sure, I can see 6-8 fights per day.