A sluggish toilet can be one of the first indications that a tree's root system is growing into the sewer line. The smart move would be to call a plumber to ensure that your suspicions are accurate. If tree roots are the culprit, a treatment of copper sulfate to the affected area will deliver a fatal blow to the roots. Copper sulfate will not kill the entire tree, however, because the killing agent isn't capable of moving through the tree's roots.
Pour 1/2 cup of medium-size copper sulfate crystals into the toilet. Flush the crystals and wait 10 minutes. Flush a second application down the toilet.
Exit the house for the rest of the day to prevent breathing in fumes. Open windows to allow for air circulation when you return.
Flush the toilet to see how it is working. It generally takes multiple applications of copper sulfate to improve the plumbing and kill the tree's roots.
Reapply the copper sulfate every day for two weeks. You may have to wait for the dead roots to break off of the tree to see improvement in your toilet's flushing capabilities.
Flush copper sulfate down the toilet every few months to ensure the roots do not grow back.