I'm renting a house,and ever since I've noticed the toilet has always flush slow. I looked at the cleanout and notice that the cleanout sizes are different. I'll explain it like this .Say your looking at an upside down "T" opening to cleanout is 4", Connection to Iron pipe 4" but coming from house is 1 1/2 ". Toilet Flange is 4",so is Iron pipe,But then it goes under ground where I can't see it,Do you think someone could've connected it,or reduced the 4" to 1 1/2 with a reducer from the toilet original 4" ?The cleanout sweeps in the direction towards the city's main sewer line .
if the toilet outflow pipe was reduced somewhere along the line to 1 1/2 you would have more than a slow flush.you would be having problems BIG TIME.