PC's spending too much time roleplaying with unimportant NPCs.
Doesn't their choice to roleplay with them
make them important?
I feel like this is simply a problem of having too hard of, ahem,
fixed-path adventures rather than allowing players the freedom to choose what is important to them and what is mostly set-dressing. If the players find some random NPC just too fun not to like, that's a clear signal to me to
feed that genuine enthusiasm, not dismiss it as a boring annoyance that they have not gotten with the program and stuck to the ra--er, the
right NPCs.
Another vote for combat. As a DM, if a battle scene takes longer than an hour, I'm bored with it. As a player, if it takes longer than 5 minutes between turns, I'm bored.
This has been true for decades, across every edition I've played, and across all levels.
Conversely, if the rounds are too
quick I get bored. Quick rounds mean nothing actually interesting happened, it was just "I hit it with my sword" "you deal damage" "I throw a knife at it" "you miss" etc., etc.
ad nauseam.
I'd much rather a battle where a person needs to make decisions. Yes, going too long is also a problem, but too short and battle is easily flowcharted. The moment I realized that that was what was going on in the old-school style fights I'd played, I finally understood why I hated "ultralight" combat stuff so much. There was no requirement for creative thought because 99.9% of situations could just be fed into a simple like ten-node flowchart.
Or, to put it into your terms: I realized that what I chose to do on my turn was basically fixed, so for the 10-15 minutes even a quick OSR-like combat would take, I'm bored out of my skull and desperately hoping some interesting roleplay happens to alleviate it.