Earlier quoted context omitted.
Some cautionary comments: Often your advisor in grad school will force you to focus on what they want rather than what you want to learn. This is less likely in a math department, but more likely in a CS one. As with everything, it all depends on your advisor. The PhD is replete with hoops you have to take that will be orthogonal to your goal of learning (e.g. spending a lot of your time doing HW on the professor's p…
While everything else you say may be true, I take issue with efficiency being a function of age. Fundamentally you have no idea how many years you will get. Be efficient at all ages, and live like it might (and might not!) be your last year, regardless. Personally I’ve had close brushes with death a couple times in my life, and the dice roll could easily have gone the other way. I’m sure I’m far from alone in this.
Also, those in their 20s often haven't lived long enough to know how to gauge their efficiency. The person who has spent decades working 40 hour/week jobs knows better the value of less free time.