KLS wrote:

We've got two American Standard Cadet toilets. We bought the first
about 2 years ago and are still very pleased with its performance. The
second one was bought about 3 months ago on the strength of the first
one's good work at removing waste, but it is not working nearly as
well.

The flapper seems to be closing too soon, not allowing enough water to
take away the waste.
SNIP

The flapper is supposed to "float" , allowing it to close later.
I would replace the flapper. As a test, swap it with the flapper
from the other toilet.

The sink drain being slow is probably un-related./

Jim