Slow and neat has never failed a wall though. Tacking stuff up and "trueing it later" works until it doesn't, like when you get one plate that's crowned the wrong way and now your whole corner is racked a half inch and you're fighting it with a sledge. The 45 minutes is nice until the day you sheet it and find the diagonal off by 3/8 and you're pulling nails for an hour. I've seen guys frame fast and I've seen guys frame right, and the overlap isn't as big as people act like. Also, some random old dude watching you in your driveway is not the same as a code book.

A 6 inch knife flexing is normal though, that's just how those blades are made. I ran into this same thing about 12 years ago and a guy told me the same stuff, so I went and bought a stiffer knife and honestly it didn't fix anything. The real problem was my pressure, not the blade. I was pushing way too hard on the first coat trying to bury the tape in one pass, and that's what was causing the hollow spots, not the flex. A stiffer knife just hid it for a while because I couldn't bow the blade as easy, but I was still loading up too much mud in the middle. Once I backed off and started running two thinner coats with lighter pressure, my joints came out flat with the same flexy knife I'd been blaming for months. Not saying your guy was wrong about what he saw, but the tool gets blamed a lot when it's really the hand holding it.