We had a similar problem with our Duravit toilet. We followed the instructions above, and after pulling out both the intake water controller portion, as well as the flush control, he didn't find any calcium build-up: just some rust stains. As a final check, my husband reached down into the toilet tank and felt something stuck in the bottom stopper valve. He felt what he thought was a broken piece of rubber, and after tugging on it with some force, he pulled the foreign object out in his hand from the access hole in the wall, and found a perfectly intact, but dead scorpion]2 It must have died just that morning when the toilet started running. Luckily he wasn't stung.

Slow flushing is not a symptom of a clog in the main line, that's not how toilets work.

You are probably thinking the pipe is full of water, so the toilet is having a hard time adding more, but if your pipe was full of water you would have water coming out of your bathtub! And every sink downstairs would be full of water.

A slow flushing toilet is simply an old, bad, toilet. It has nothing to do with the sewer line.

A sewer line is empty usually, unlike supply lines, sewer lines do not normally having anything in them, except for the few seconds after use. Even if the main line was slow (which it probably isn't), the small amount of water in the toilet would simply fill up the pipe above that part at a normal speed, then move down a bit slower - you would notice nothing.

Get yourself a new toilet, I like the Toto brand, you can buy one on Amazon for about $200.

If you had a clog the symptoms would be different: When you started using the sink it would work completely normally. Only after you ran the sink for a while (5 - 10 minutes even) it would start to slow down, and then it would stop draining and the water would just sit there. You would wait a long time and that water would very slowly drain down.