I do tend to cook like this, but that's because I'm an inexperienced cook :-) I don't trust my ability to complete a prep task in the bounded amount of time you have once you've actually started cooking, so unless the time available is clearly long and the prep task trivial I tend to do most to all of the prep in advance. You're absolutely right that it takes longer in wall clock time but I find it less stressful, and that's valuable to me.
This is good sensible planning, and it can shift as you get more experienced. You'll recognise orders that make sense (I know I need the oven preheated so start that now, then chop / I have a wait before this step and just need to chop the mushrooms which I know only take me X so I'll do that then).
