Here are a couple of quick thoughts on max deadlifts.
1) How long you stay down before the pull is very individual. Some do best when they learn to pull quickly and minimize the time spent in the bottom position, while others do best when they take their time at the bottom and wait for everything to feel right. Experiment to find what works best for you.
2) Form is dependent on the goal. If max loads are the goal, you may benefit from learning to round the upper back, but if back health is the goal, brace the neutral spine and maintain the normal curves.
If you’d like to watch someone who has mastered the art of upper back rounding, check out Konstantin Konstantinovs here:
In an interview, he’s mentioned that once he learned how to round his upper back he was able to take his deadlifting strength to the next level.