I already know the answer, but I'll still ask my question because you're the sort of person I've been referencing. What joy!
So, question is this: What is the downside for a player to improve so that that "decently quick enough" becomes "quick enough because did my part?"
As a follow up, let me give a hypothetical. Bolding for visibility since this is the main part of the post, really and I can't figure out how to do a line break on a forum layout that hasn't been updated since 2012.
Say we have... Oh, I don't know. A Dragoon player. They go into Ramuh normal. They do literally nothing for 40% of the time (Ramuh normal is a 100% uptime fight, by the way). They keep Chaos Thrust, a natural part of the rotation that has 100% uptime by default... Oh, I dunno. Let's give a random number of... 29.07%. In spite going down the path with Disembowel to have that percentage of uptime, they still have... Oh, let's say 38.33% Disembowel uptime somehow. They also failed to use their CDs, save for high jump.... 40-80% of the time. Finally, out of Nastrond uses, this random, totally hypothetical Dragoon only used it 8 times, out of 18 possible uses. This super mega hypothetical Dragoon did NOT die. Please keep that in mind.
Would you say this Dragoon, who LITERALLY DID NOTHING FOR HALF THE FIGHT, carried their weight? Did their part? Helped that... Oof, that was a long HYPOTHETICAL Ramuh normal... But helped it go "quickly enough?" Please, give me your insights. =)
To address the second half of your post: 70% of the end game dungeon runs I've been in have had 1 or more of the following qualities: Single Pull tank, Zero DPS healer, non-AoE DPS, any of the 3 that clearly didn't know what they were doing. By the latter most, I'm not talking about "missed a CD here, missed a positional there." No, I'm talking on the scale of Black Mages using Transpose end game. Like, as part of their rotation. I had 2 of those in a row. They were indeed different Black Mages. Their AoE rotation was to spam freeze. The healers did more then them.
If any of those things are present, the dungeon run isn't going smoothly. 70% is a pretty large chunk, eh? Gets worse if you look at leveling 71-80. Oh boy. I leveled an Alt and a different job and I can say that I've seen some things.