Most toilets seem to have a venturi type effect so that the contents of the bowl are "sucked" out. Ours doesn't. Tissue paper is often left floating though thankfully heavier content does tend to go away.
The "S" bend is all part of the toilet bowl structure so there would appear to be no potential here for air leaks.
The level in the bowl doesn't go up so it would appear that there is no blockage.
Any ideas?