I've found that being energy constrained is much easier to track than time constrained. You notice instantly when a task takes a ton of your energy, you start hating the task so you work hard to ensure you don't have to do the task any more. Compare that to someone who mostly wastes time, they'd happily sit and waste tons of time every day since you don't really feel your time slip away.
There are many physical and psychological issues which diminish people’s mental energy. ADHD and depression being the most obvious.
Fortunately for many of those conditions there are also ways to increase the amount of mental energy you have available.
While I cannot speak for anyone but myself, my performance on highly challenging tasks is capped at 4-5 hours per day. At that point, it is better for me to switch to lower hanging fruit.
If I am feeling especially inspired, sometimes I'll put in a 12+ hour day of hard work. Rarely, even two or more in a row. But inevitably, I will feel extra burned out in the subsequent days.
I am typically energy constrained rather than time constrained, as I imagine is the case for many working in engineering/science. Yet, the author's advice remains useful. Deliberate practice should provide gains to both time and energy costs of tasks.
For me most productivity advice, including this post, ultimately reduces to "try to get better at your trade, and track your progress".