I've personally never had a problem like this yet, but then again I haven't done any of the 50 dungeons. I've run into the opposite problem on occasion, when two of us talk over each other to explain fights

I think by now the lower level dungeons (what I'm doing, both by myself and taking a lower friend through them) have become pretty much empty of people rushing to 50 - they've already finished the story, done the dungeons, and are fate grinding new classes. Anyone still doing the story is either doing it slowly, at their own pace, and therefore won't rush, or doing it to help others, and therefore again won't rush.