Players who don't know their characters and/or don't have everything readily available. It just drives me batty having the action slow to a crawl while a player looks up what spells they have prepared or what their attack modifier & damage are.
I have several friends like this, and they often have to talk it out each time. Could have several attacks in a round, each attack is:
*ROLL* "Rolled a 9, plus three for dex is 12, plus three for proficiency is 15. Did I hit? Oh, magic quarterstaff for 16. Did I hit now? I did? Great." *ROLL* "Rolled a 6 for damage, plus three for dex is 9. What's that, the magic quarterstaff? Add more more point of damage."
Not my number one pet peeve, but it's so easy to speed up.
One of my biggest pet peeves is when a player is incapable of re-deriving their bonuses on the fly.*ROLL* "Rolled a 9, plus three for dex is 12, plus three for proficiency is 15. Did I hit? Oh, magic quarterstaff for 16. Did I hit now? I did? Great." *ROLL* "Rolled a 6 for damage, plus three for dex is 9. What's that, the magic quarterstaff? Add more more point of damage."