That changes things, and makes me skeptical that there's a venting issue that's suddenly changed that's causing the problem. Everything else dumps into the 4" line, I assume, and none of the other fixtures are slow or gurgling, etc...
Accumulation of sludge in the line that's causing slow drainage is still an option, but less of one.
How about a sag in the line? Is there a low spot on that 2" sink drain that could be causing the problem? Sludge will collect there and cause a restriction. If it's deep enough, it acts like a second trap and can trap air bubbles in the line that restrict flow.
Did you take the trap completely apart and make sure all those lines are 100% clear?
IME, on a 2" stub, the sink should drain super fast, even without a vent.
