I always specify "speed run" or "fast run" or something similar if I'm planning to zerg through a quest as quickly as we can.
When I join someone else's group, I don't assume this will be a zergfest unless it says so on the LFM, or the leader tells everyone it will be.
Recently I posted an LFM for the Shadow Crypt that read, "Elite for BB. All welcome. IP." A couple of people join just as I start, after the fight in the first room, so we start together. Shortly thereafter a ranger joins and after about a minute, tells me that my map sucks because this is taking way too long. A couple of minutes after that (when he was apparently just wandering from room to room) he starts telling me how much of a noob I am because "this quest should take five minutes" and then he quits. At that point I believe we'd been inside for eight minutes.
I was going for all the gears and was using a map. At least one of the other people in the party had never done it before, so I was explaining it on voice to them as we went (which wasn't really slowing us down at all.) I think our final completion time was 19 or 20 minutes.
He sent me several tells after he ragequit, letting me know I didn't know what I was doing because that quest should only take five minutes. I didn't respond to his tells.
Do you assume that all LFM's are zerg runs unless they specify "NOT a zerg run"? I never do, but I've run into more and more people lately (very few of whom are actually rude about it) who get frustrated/annoyed whenever they join a group that isn't a speed run.