The Slooooooow Flush
To figure out what's causing your slow flush you need to know whether water isn't getting into the bowl fast enough or isn't getting out fast enough.
Take a bucket of water and dump it quickly into the toilet bowl. If it flushes normally (like when you flush a working toilet), water is not getting from the tank INTO THE BOWL fast enough and the problem is in the toilet tank. If it doesn't flush normally, water probably isn't getting OUT fast enough and the problem is in the house plumbing. .