Edit : read more posts and my initial post seemed irrelevant.
Are their dwarves drinking from a high percentage of the barrels at once? You said 50-60 barrel stockpile and at least 50 dwarves drinking. That means there's other dwarves waiting to get a turn, getting even thirstier while they wait. Hence, a queue is forming of very thirsty dwarves.
If they all have blue arrows it means they can't get the drinks fast enough, not that they are drinking. Often you'll see a dwarf drinking and no blue arrow, I assume this is when the dwarf go to the barrel when his thirst was not too high yet. The guy going to the well is also a sign of too few barrels. Try have 100 barrels at least, so there's no wait.
Guys who have to wait to drink are becoming further dehydrated, so maybe the single drink isn't filling them all the way up. They work slower if they drink water, then may have to drink again sooner than otherwise. You should have at least 10 times the dwarves in alcohol servings, probably 20 times (at least 1 11x11 stockpile - 1 barrel per dwarf is the failsafe)
I go up to 200 dwarves, never get more than 20 drinking at once.
Also note: when dwarves are spawned, their drink level is set to default, so your initial 7 all get thirsty/tired/hungry etc at the same time, as probably do migrant waves etc After a while this goes out of sync, due to different movements of the dwarves etc but it's a phenomena which does exist, so you need to cater for peak consumption.