If it's vent related you can check this, if you can access the bottom of the vent pipe that goes through the roof.
My vent pipe is galv. pipe, over the many years rust developed and as it fell to the base where there was an elbow into the sewer line it became clogged. I was able to clear it with some snaking and a shop vac, also ran a piece of emt down the pipe from above to break up the blockage. While the sink never backed up too bad, it flows freely now. and the toilet flushes better. You never know what you'll discover in an old house.

Take your plunger and plunge it when it slows down.
should clear most anything. I found a toothbrush in our pipe once.