Can't see what is going on out of the picture, but slow drains are not caused by venting problems. Plumbing fixtures are self venting. The system vent prevents the traps from siphoning dry. Your slow drain is caused by a clog. The hair is your problem. Try one of these: http://zipitclean.com/2009/08/zip-it...nt-believe-it/
This DIY you--tube production is NOT an exaggeration. I use these and you often get just what he shows in that video. And I will give you even money that the guy had previoulsly put a couple gallons of drano through over a period of time, with almost no effect on that loch ness monster!