>As I started writing code for this problem, I found out I could work on it for hours on end without distraction, quite unusual for me. I chalk this up to a tight feedback loop. Have an idea, implement it, get back a number. Think of ways to improve the number, start the cycle all over again. This is a cycle I would run dozens of times a day. Obviously more fundamental ideas would take more time to see the fruits of, and that's when I actually lost focus, but when the brain has been rewarded so richly with small cycles, it can afford to go a bit longer without reinforcement. This tells me that A/B testing or a similar numeric optimisation area would be quite motivating to me and I've made a mental note to go into this in the future.

I've had this experience before. I have a severe problem of not being able to focus on anything, and it feels great to be working on a problem like this. I'd love to find a problem like that again.