A couple of years ago I read Warren Buffett's books and some stories about him. One of the lessons stuck with me, it's something I think about regularly when deciding what to spend time on. Below is an excerpt from James Clear's website.
The story of Mike Flint
Mike Flint was Buffett's personal airplane pilot for 10 years. (Flint has also flown four US Presidents, so I think we can safely say he is good at his job.) According to Flint, he was talking about his career priorities with Buffett when his boss asked the pilot to go through a 3-step exercise. Here's how it works…
STEP 1: Buffett started by asking Flint to write down his top 25 career goals. So, Flint took some time and wrote them down. (Note: you could also complete this exercise with goals for a shorter timeline. For example, write down the top 25 things you want to accomplish this week.)
STEP 2: Then, Buffett asked Flint to review his list and circle his top 5 goals. Again, Flint took some time, made his way through the list, and eventually decided on his 5 most important goals.
STEP 3: At this point, Flint had two lists. The 5 items he had circled were List A and the 20 items he had not circled were List B.
Flint confirmed that he would start working on his top 5 goals right away. And that's when Buffett asked him about the second list, “And what about the ones you didn't circle?”
Flint replied, “Well, the top 5 are my primary focus, but the other 20 come in a close second. They are still important so I’ll work on those intermittently as I see fit. They are not as urgent, but I still plan to give them a dedicated effort.”
To which Buffett replied, “No. You’ve got it wrong, Mike. Everything you didn’t circle just became your Avoid-At-All-Cost list. No matter what, these things get no attention from you until you’ve succeeded with your top 5.”
What I don't spend time on
I think this is great advice, it's something I think about regularly. I will list 2 things here that I decided not to spend time on after reading this Warren Buffett bit a few yeas ago.
Quantum Field Theory (and research-level physics in general). I have a degree in Physics, it's a second degree I got mostly for fun and curiousity. Getting it has been one of the greatest decisions of my life, learning and doing Physics has been continually paying off ever since than, both personally and for work (Computer Science + Physics = Data Science). As a physicist you learn the deep insights of your predecessors, but also that with certainty they are wrong in some sense, and over time their achievements will be "just" an approximation or a special case. So physicists are always learning and adjusting their perspective. Physicists also have a keen sense for measurements, statistics, errors, which is very useful when dealing with numbers in the real world.
So after I got the Msc degree in Physics back in 2008, I started a Phd that I never finished because I went off to do a startup. Maybe it's because I never finished it, but for many years afterwards, I had this romantic notion and kept going back to Physics in my free time, reading papers, books. I once even had a yearly goal of writing a paper and submitting it to a journal. I actually did it, I wrote a short paper titled Pure Lattice Gauge Theory in the Expanding Universe, and submitted it to the Physical Review Letters (PRL), but got rejected (most papers get rejected at first, real scientists keep improving and submitting). Since I don't really care about getting into a journal, I didn't spend any more time on it, now it just sits on Arxiv .
So, when I decided not to spend time on Physics anymore, it was not a trivial thing to do. I really like to do Physics, even in my spare time. But at the end of the day, I'm not in Academia, so I don't actually have time to follow topic(s) of research like real scientists do and figure out how to contribute. Also, academia is pretty crowded, and research topics have become very specialized. It's not really a good investment of time for me; investing my time into things related to software/startups has much higher potential impact and payoff. I also stopped reading Physics blogs, all the drama around string theory is fun but ultimately just a distraction.
Haskell. I first came into contact with Haskell in 2013 when working at Prezi. There are a lot of things that are intesting about Haskell: it's a strongly typed, purely functional language with type classes, type inference, etc. You can do really cool things with Haskell, or so it seems at first. In 2013, before systems like Airflow were opensourced by Airbnb, there was no good standard open source ETL system; but we needed one at Prezi, to replace the bash hairball we had. We used this project as a testing ground for Haskell, and wrote an ETL framework in Haskell (I think we called it Datapipe). We spent about 3-6 months on it and it was a big disappointment. Although we put it into production at one point, it was quickly replaced by a re-write (this time in Go, which I believe is still in production). At a later point I tried using Haskell for a personal project for representing Physics equations and quantities, and also ran into major/deep problems (oddly, I found it easier to model what I wanted with C++ templates of all things).
Despite all these failures, I continued to be interested in Haskell. Although I adopted a critical stance, and usually argued against it in conversations, I assumed "it's just me, I don't "get it". But I never found or really saw good reasons and generalizable examples where using Haskell in production really made sense. So when I read the Warren Buffett bit, I knew that Haskell is one of those things that I have to stop spending time on, it's just not a practical thing for me to use. I share John Carmack's stance, he views Haskell as a good source for ideas to use in imperative languages like C++.
I don't think spending time reading/writing Physics papers or Haskell has been a waste for me. Far from it. I learned a lot doing these things. But I've also determined that investing more time into them is not worth it right now. Remembering Warren Buffett and his pilot is a great way to remind myself to stick to it, and spend time on other, potentially more impactful things .
 A PRD paper later referenced it and called it "pioneering work", but I'm pretty sure it's not.
 Having said that, if I were to go on a sabbatical, doing Physics would be fair game. The point of sabbatical is to take time off from the normal pursuit of things and spend time on more risky/fun projects, in a timeboxed way.