Return on time, (ROT?), is always something other than time and thus measures of it are highly subjective. In the end, what you are spending is your life and although there might be preferable ways of doing it, I'd imagine that being the achievers they are, HN members usually err on the side of investing it. Actually PG's essay offers a nice exposition of spending time (having fun) vs investing it vs fake work:
And that's leaving aside entirely the idea that time == money (ie. you can often substitute one for the other, or invest one to later gain the other).
Unless you are doing something to prolong your life (eating healthy, working on an anti-aging cure etc). I don't think the analogy is that bad.
Investing time understanding users or improving your tools makes a big difference in the long term. When you understand your users you can work on the right features. When you sharpen your tools you spend less time wasting time with repeat tasks. For example, how long it takes to release a new version of your product matters a lot. Automated testing for most common problems makes you move faster.
In general, the best way to invest your time is to automate repetitive tasks.