That's more because:
1) Speed runs burn people out
2) Speed runs annoy people who don't like playing the golden child jobs that are "required" by a good number of groups
3) Dungeon design is very uninspired between lack of bosses and the fact mobs inside are leashed (believe me, if you were forced to actually fight the trash mobs in any of the instances they wouldn't seem so short to you)
4) There's very few dungeons, even for a game with a lv50 cap
#3 is really the big one, with #4 as a close runner up. There's not enough dungeons to actually support the level climb. #1 and #2 are also factors behind boredom. Especially when you're going up against a 0.000000001% drop rate on anything worthwhile.
I mean, for example, AV would probably be played differently if it had a wing with just Garlean forces, and the timed objective would be finding the captive the Knights of Ishgard refer to in the "What Glitters Always Isn't Gold" quest. You zone in and have, say, 30 minutes to find the captive. If you take too long the captive is killed and you miss out on one boss. Have a total of five bosses. Include a trap of sorts that locks you in the room with the gold gas that does damage over time to you until you kill a boss in there. Then coincounter. Then a garlean officer of some sort. Then the guy that's there solely to execute the captive. And then Miser's mistress. Each with their own loot table to encourage the full clear.
Between trash fights and running around (specially if you're going for the timed objective, which would not be the end of the instance), AV could then take longer than the 20-25 minutes it currently takes for the speed run. Could even add zone drops from the trash mobs as a bit of a bonus.
Then it'd be a matter of balancing the mobs and of course, balancing the classes so that most if not all group comps can do the timed objective once they know the dungeon and get a little practice with it.