Depending on the content being run... the difference between (for example) one DPS using single-target attacks vs AOE attacks on trash over the course of an entire run... stretches a 13 minute dungeon to about 15-16 minutes (in my experience).

Same with healers. If a healer is overhealing and not leveraging their DPS capabilities, maybe another minute on top of that. If a tank isn't AOEing, the party is probably dying since the trash will destroy everyone else.

I've been in groups where 3/4 of the party has the raid weapon in week 2 or 3 of the raid cycle... and while the clear time for something like an expert dungeon is faster, maybe we are talking 30-45 seconds faster. Which is nice. But in the greater scheme of things, a party that has no deaths on bosses will do better damage than a better-geared party where someone does die to mechanics that they should have done properly.