Slow toilet; drain or vent?

One of the toilets in my house is draining slowly. The siphon forms and the bowl empties completely, including waste solids, but it takes longer than it used to. I can't believe that the fixture itself is clogged: I live alone, and the only things that go in the bowl are human waste and toilet paper.

The other toilets in the house function normally; so it's not the horizontal sewer line in the basement. The other fixtures near the problem toilet (shower & double sink vanity) function normally - with the exception that I've caught an occasional whiff of sewer gas from the sink drain right after flushing the toilet.

I've got a bunch of vent stacks in the roof. Near as I can tell, there's one that serves only this toilet and the double-sink vanity. The shower seems to have its own vent, as do the other bathrooms, kitchen and laundry.

I'm inclined to suspect the vent. Can an obstructed vent slow a toilet like this? Is there a way to check it, or some kind of other indication I should be looking for?