I think the battle mat is the biggest time-waster at my table.
Expectation: Ah, I see. This clear representation of the battle scene really helps us imagine the moment.
Reality: Player 1: Ah, so the rogue moves over there then? In that case, I'm not going to cast a fireball spell, instead I'm going to completely change my entire action and move to a completely different area, and cast a different spell.
Player 2: Oh, so if the rogue is going to move over there, and since the mage isn't going to cast fireball? Well. Hmm, let me see...no, my first idea won't work, so instead I'm going to step over here, one, two, three, four, five...wait no, three, four FIVE six squares, and then I'm going to attack that orc.
Player 3: Well if the mage isn't going to cast fireball and if the fighter isn't going to be adjacent anymore...hmm. No, let me re-think my entire round.
Player 4: Me too, the battlefield is completely different, I need 15 minutes to recalculate The Best TacticTM. Where did the rogue move to?