I don't see how the Einstein example is relevant, feel free to elaborate.

On instagram... you're introduction valuation, not nominal productivity.

i.e., imagine I can either produce 1 smartphone after a year working 100 hours per week, or produce 100 smartphones after 10 years, working 10 hours per week.

Well, nominally, the latter example is way more productive. You produce 10x as many phones per year while working 1/10th the time. There's absolutely no way you can say that the former is more productive.

But, building today's smartphone in 10 years is obviously going to leave you with little value. In 10 years, your 10-year old tech will be valued very little.

Your instagram example is like that. The engineers working on it may have been much more productive if given 10 years to work on their project, but its valuation would be shit because it's last to market instead of first to market. So, not a great example.

If you're trying to say that working hard in short periods, allowing you to get to market before anyone else, is better than taking your time, then obviously that's true, but that's more a function of competition than it is of productivity!

Further, it's a poor example. Show me data that's meaningful for a large group of people, not extreme outliers. How many people do you know that created a $1b company or came up with g relativity theory? It's like saying 'dropping out of school is a great idea, look at Gates and Zuckerberg'. When in reality the data overwhelmingly supports the notion that finishing school has positive outcomes in your life. You can prove anything by looking at selective (outlier) examples, but show me examples that commonly occur.

Meanwhile overwhelmingly studies show that productivity drops sharply after some amount of hours. You get more work done per unit of time when working 8 hours per day, than 16 hours, period. That's what productivity measures.

There is obviously some lower bound, if you work in 3 minute intervals you're spending a lot of your time 'starting up' your workspace and getting your brain tuned in your work, and so productivity (work per unit of time) drops. I wouldn't be surprised if 4 times 90 minutes was less productive than working for 6 hours, that makes a lot of sense. But there's also an upper bound, where you lose concentration and motivation and are better off resting and getting back to it the next day, such that working two days of 6 hours is more productive than a single 12 hour session. Feel free to look at the many, many studies that confirm this. The above applies to virtually anyone, any organisation, any individual, productivity has diminishing returns as you scale way down or way up.