I have this exact same problem. I hope it's ok to post in here too, as I would just be duplicating exactly what the originaly replier entered
Locally, everyone keeps telling me something is stuck in the trap under the toilet where it mounts on the floor. But this toilet has done this for years. If it something stuck somewhere, the problem would have to get worse wouldn't it? The toilet never stops up or backs up. It just takes several flushes for everything to go down. I can pour in water in the bowl from a bucket, and it will flush automatically completely.
I have a new flapper and float in the tank. I have set the water level almost level with the overflow tube but it doesn't change the affect no matter what leve the water in the tank is in.
I flapper closes just as soon as the water level in the tank drops below the level of the flapper itself, so I assume it's closing fast enough.
I've never removed a toilet before but I have seen it done. Could it be that waxy donut type ring under the toilet not causing a good seal or something?