It was not directed at you, sorry if you felt it was. it was at the people who designed and built the faulty product.
I have no idea where the 1.6 toilet came from that I personally replaced with the 5 GPF unit. It was installed before we bought the house (although not long before. the bathroom was a compete remodel before it was put on the market). I might have worked better if they replaced the run in the basement but with our septic the sewer line does not exit the house at floor level in the basement. It’s about 4 feet up. That does not help with the slope of the drain pipe either.
Think roughly 1/8th to 1/4 inch fall per foot as correct, in general, that looks almost flat to the normal person.