I had something similar when I moved into my place. Turned out to be years of gunk buildup, just tons of hair and old soap and stuff clumping together. It's gross but not uncommon. Got a plumber to take care of it, drain runs perfect now. Good luck, man!

Wow, what a strange place for a 'Do Not Enter' sign. I can't imagine how it would fit into any sort of home aesthetic. Definitely take it down! On another note, hope you're enjoying your new place aside from the odd sign placement. :-)