I'm pretty sure I said the rounded lower back was a problem, and that he needs a flat back. I'll clarify though. The flat back takes priority over speed. That has to happen.
That being said, teaching people to lift aggressively as beginners is important. So many beginners focus on slow contractions, and on most lifts, this is wrong.
For me, my deadlift form is compromised when I don't try to rip the bar up. If I lift the way Night suggested, I'm significantly more likely to get my hips out of position. The quicker I can break the bar off the floor, the better. I stay more upright this way. I'm faster than I am strong, and I have to use this to my advantage. My initial burst determines whether I complete the lift or not 99% of the time. Not everyone is the same, so to each his own. I simply wanted to point out that there is more than one way to approach the deadlift.