The bathroom sink drains VERY slow - sink fills up just from hand washing/toothbrushing. Takes several minutes after to empty. Bathtub & toilet drain fine. Trap is clear. Used drain-O & plunger, no luck. Could it be a vent issue if just this one fixture is affected? THANKS!

Reach under lavatory and loosen the nut holding the lift rod in place. Pull lift rod back about 1". Pull stopper up and out of drain.
Clean hair out of drain with straighten wire coat hanger. Put stopper back in place. Make sure it is turned properly so lift rod passes through it.. Tighten nut around lift rod.