"gogogo" might make me quit this game.

[Yeol]

From my understanding, this is less about pull size and more about speed.

OP, people are fast because it is the most effective way to maximize their reward. The less time spent to get the reward the better. This is human nature. They want the fastest way to achieve something.

Just make dungeons harder. Make trash hit harder and have more health. Problem solved. If players only have one option/speed to go through dungeons we won't have to be so divided on how to proceed.

Dungeons are easy by design. They have to be farm-friendly. People are going to be spamming dungeons for EXP. Leveling multiple jobs. Some will be spamming them for dailies (Expert dungeons). You don't want spammable content to be hard.

Players falling behind could be due to many reasons:
1- They are slow at turns. They don't know where the next turn is or the exit to the next room.
2- As a caster/ranged they always stay very far behind even when attacking. They can step closer when they see mobs/bosses dying. So they can move with the rest of the party to the next pack.
3- Not using sprint properly. Running into walls or running sideways a lot instead of a straight line. They're not using it in alignment with the tank, but instead using it as a catchup button.

These are what I've mostly noticed with slow players in my parties.