In the old house the pop up in the bathroom sink would unscrew. With that off, I used a length of metal clothes hanger which I put a small hook to the end. I used the to fish the long hair and soap scum out that had built up on the internal pop up riser mechanism. Followed that with Liquid Plumber. In the new house the pop up does not unscrew so I just use Liquid Plumber but more frequently than in the old house. I doubt I'd have this problem if I had a sink that used the old rubber plug instead of the pop up system. As mention previously, as a last resort you can just remove the grease trap and clean that out.
Bob
Bob