This can be true under certain circumstances. I do think 15 minutes a day of meditation is probably a better use of time than an hour a day for most people. But for many common human activities there are increasing marginal returns to time spent, because spending more time allows you to acquire expertise. This is the reason people specialize in their professional lives. In intellectual endeavors especially, most of the benefit comes from doing some particular thing better than (most of) the competition. Dabbling in lots of different skills will sometimes get you there (by allowing you to combine skills in a way someone else can't) but the straightforward approach is to focus on your strengths.
As a rule of thumb, I'd say that "support" behaviors like exercising, meditating, cooking, planning, socializing, checking the news/facebook/whatever, those have diminishing returns. Do a little, gain a lot. But something that falls into your core competencies (studying a subject you plan to get good at, for instance) has increasing returns, so a good strategy is to carefully choose a small number of these activities and dive in wholeheartedly.
I agree with the thrust of your argument since it is true for some things, but I disagree on the issue of meditating. Most of the cool stuff happens only with large amounts of dedicated practice, such I believe a person would be better served by, say, 10 days of 10 hours of meditation each than 400 days of 15 minutes of meditation (100 days of 1 hour of meditation would still be pretty good, maybe even better than the 10 days, but for different reasons than why the 10 days is better than 400).
To me this suggests that it may be hard to know when it is appropriate to apply what you notice here and when not.
Most people do not have the discipline to do just 15 minutes of something. They have to do 0, or 45. This is your actual competition.
Then try doing something once a week, for an hour.
60 minutes once a week may not be as good as 10 minutes a day, every day, but it's a start.
Epistemic status: A mental piton, to hang your climbing rope on
The second half hour of cardio you do in a day matters less to your overall happiness and health than the first half hour you do, the second quarter-hour, the first quarter-hour; knitting this into our base mental framework is why the law of diminishing returns is such a "Wow!" moment for a lot of beginner econ students.
The problem with consistently applying the law of diminishing returns to everyday life, however, is that it's boring. People don't become socially-renowned Zen masters through 1 pomodoro of samadhi a day. All they get are improvements to their overall mood and the occasional boring state of pure pleasure and equanimity. People don't become [in]famous computer hackers by spending 2, not-especially-well-planned hours a week learning how to automate the boring stuff with Python; they just learn enough to be a little dangerous.
It doesn't feel like you're using a glitch in the Matrix disguised as a psuedoscientific law. But that's exactly what you're doing.
Understand: Most people do not have the discipline to do just 15 minutes of something. They have to do 0, or 45. This is your actual competition.
You can routinely beat the average, in almost every everyday domain, by applying the law of diminishing returns to your own life ruthlessly and consistently. Or perhaps I should say, ruthlessly and consistently enough?