The mind craves novelty. Part of why problem solving is satisfying is that it gives us new perspectives or new results. When we're bottlenecked by knowledge or resources it's easy to become burned out and to 'cheat' with a wandering mind seeking information feeds or hobby interests.

I get best results starting with the hardest problems first thing in the day, and as complications or dead ends manifest, calculate if it is best put off until something gives (or the mind thinks up something randomly in the commute or shower), and move on to intermediate issues, repeating until the end of the day are small trivial tasks and maintenance.

This is the daily grind though, project deadlines or resource intensive efforts are going to skew individual results.