I think there has always been a variation. I seem to recall that the old cistern upright with a chain flushed far better than the current low level ones do. Its hardly surprising since the head of water is going to be more, Given the bore of the pipes and all other things being equal, but the tendency to use less water nowadays has not helped either. Its a simple thing is it not, you need to be able to get enough flow to make whatever is in the bowl flow over the bend and hit the downpipe. I've never myself been to Japan but it seems over there there are many innovative forms of toilet, and leaving aside the more frivolous heated seats etc, many seem to have power flushing so called auto cleaning and other innovations, but we stick with the same old basic design I've seen since I was tall enough to look over the edge.
Of course there are these special ones that mash it all up and dissolve it and then pump it away, a bit like a supercharged chemical toilet, but in the main, nobody seems to have done much. Its still a pain getting the limescale off and it still pongs even if it looks clean. Brian