Why is my sink not draining well?
Remove the pop-up drain - some just need a quarter-turn to disengage, some you have to fuss with the rod that pops them up, some can be either way (and end up as quarter-turn when I put them back together - I suppose the other way is for making them harder to steal in public bathrooms.)

There is probably a (revolting, sorry, them's the breaks of plumbing) mass of hair and glop hung up in the stopper. I typically need to do this every year or so to our main bathroom sink as hair does get in there, and there's a point where trying to prevent that completely is more work than just cleaning it out when needed.

You may want rubber gloves, depending on your personal tolerance for revolting drain glop.