They seem to reduce flush...imagine your toilet waste is half blocked. Maybe not half blocked but not fully open as it was designed.
Although saying that I have installed 3 new toilets with push buttons and the cheaper the toilet the worse the flush seems to be no matter how high the water level is. I dont know what my advice would be as there could be many contributing factors. The cistern holds too little water, the path the water runs down the pan is slowed by design, the siphon could be opening half what it should be. A bit of all of these... I'm not there so I can't see...
I have come across this problem before when using the ribbed type connector. If the fall is very good to vertical, then they seem to work ok. If the flow is anywhere near horizontal for any distance, the ribs hinder the rate of flow to a degree where matter can be left unflushed.
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