Imagine what it would be like if everyone in a full cabin, or even half of everyone in a full cabin, decided they wanted to spend ten minutes in the lav for any reason at all. You do the math. On most flights they'd have to start before takeoff.

Maybe airlines should designate one lav for people who want to go through their whole morning routine and leave another for people who just want to use the toilet and get out. That can be pretty urgent, whereas shaving, etc., isn't. Pending that, which I don't see happening anytime soon, long bathroom rituals belong in bathrooms you don't share with a couple of hundred other people who also want to use it.

I can see brushing teeth, which needn't take more than a minute, but anything beyond that should either be done well before the last hour or saved for a lounge or lav on the ground. With current rules keeping friends and colleagues from greeting people until they exit security, there are pretty much always places to freshen up before they see you. As an added bonus, you get more than four square feet of space, and the floor doesn't move. And if you think that would take too long - how do you think the people waiting for you to finish on a plane feel about that length of time?