We have one upstairs, one downstairs, both identical in all respects other than the poor operation of the one upstairs. It needs numerous flush attempts, to clear the pan contents. It has always been thus, but does seem to have become worse over the years. There is no backing up of the flush water, no rise in level as it flushes in the bowl, it just fails to flush the contents.
Both cisterns are wall mounted at waist height, both are filling to the same level, both seem to flush as rapidly - same time start to finish, but estimated.
The downstairs one goes down into the solid floor, out under the outside wall then horizontally to a manhole about 8 feet away.
The upstairs one travels on the surface two feet, sloping down to the outer wall, then T's into a vertical soil pipe, which is open vented above the roof eaves.