Toilets are designed to allow a large amount of water into the bowl at once. That water will be forced to exit the bowl just as quickly. However when the toilet becomes slow (lazy) in passing the water into the sewer main, it may mean a material buildup within the toilet itself. What the homeowner will notice is that the water level will rise slowly in the bowl and exit slowly. It’s possible that it may leave materials in the bowl – requiring multiple flushes to clear the bowl.
Over time, water minerals can form on the interior of the toilet. These deposits can be a gray or black in colour. As depicted in the graphic, these may form on the smooth glazed portions of the toilet and especially on rougher unglazed portions on the interior of the toilet.