Hi all:
It is possible that the vent stack is clogged. It is also possible that the toilet uses the lavatory drain/vent as its vent and the lavatory drain is partially clogged so check that, too.
First thing to try here, however, is to snake the toilet...

Have you used a
CLOSET AUGER to snake
through the toilet itself?? If not, purchase a closet auger and snake the drain through the toilet...see if that improves things.
After that, I would quickly lift the toilet and see if anything is caught up at the base of the toilet. If not, I would take the toilet outside, set it up on sawhorses and then pour a 5 gallon bucket into the toilet. If it drains fine then you are off to start unclogging the drain pipe itself!
Let us know what you think...
MARK