Clearing a dz is basically always faster than open world. I am struggling to think of a situation where a dz is actually slower, minus maybe a nitpick on time spent doing adds or the loading screen. The reason people do open world is because of the extra loot while useful dz content is on a lockout, or maybe a few for the competition and thrill of it.
I think that would heavily depend on the guild, but was definitely not the case for TEB. While TEB still cleared DZ's at an insane rate that would be faster than most, if not all guilds, the batphones were always stupidly fast. For a lot of the raid mobs classic-velious, people in TEB pretty much knew that if they weren't online at the time of the batphone, there is absolutely no way they are going to log in on time to make it there for the boss. Depending on which boss it was, my normal system was to pull the boss, then batphone, train boss in to whoever is already at the meet spot.
For example in classic, our average naggy kill time from spawn > batphone > dead was about 8 minutes. Vox was about 11 minutes. CT/Inny 15 minutes, and a full sky clear islands 2 to finish took about 30-45 minutes.
Kunark VS spawn > batphone > dead was under 5 minutes, trakanon and the other OW dragons 10-15 minutes, and VP no matter how much we were killing could vary from 15-45 minutes.
Velious, KT 5 minutes or less, dain 10 minutes or less, AoW cycle under 30 min, yeli/tunare 15-20 min (unless tunare had the rare occasion of stubbornness). Full clear of ToV less than an hour.
Luclin - emp 20-25 min or less, VT never more than an hour no matter how many bosses were up. If a lot of bosses were up we would often kill 2 bosses at a time.
PoP - depending on which plane we were batphoning it could range anywhere from 20-45 minutes. We even batphoned wrath council for over 3 months every spawn because we could do it in less than 25 minutes from batphone to dead.
Anyone who was in TEB could verify those times. Those times are all from time of batphone to complete, and several of them are from spawn > batphone > dead. We used to keep track of personal bests in terms of spawn > batphone > dead on batphones. Our personal fastest were all faster than the times I listed but I just put in the averages. Like for example without any poopsock, our pesonal best on naggy was 6 minutes, vox 9 minutes, VS 3 minutes, KT 4 minutes.
So yes, OW was definitely WAY faster than DZ's at least for us. But I understand what you mean by what your saying considering I've seen batphone guilds take over an hour just to kill things like naggy, vox, or even Tormax.. When I played for 2 expansions on Mischief those batphones were the slowest I had ever witnessed and prob were slower than how fast your average guild could finish it inside a DZ.