The first thing you need to do is remove the tank lid and check the flapper. Is it in good condition? If it’s worn or cracked, you can buy a replacement at any hardware store. If the flapper is in good condition, check the water level.
Often times, slow flushing is caused by a partially clogged toilet. To fix this, you need to remove the toilet tank cover, clean the rim of the bowl, and check for any hair or debris in the water trap.