The answer to 'why is my water heater slow draining' is that a slow draining water heater is often caused by a buildup of sediment at the bottom of the tank. As water heats up, minerals and other particles in the water can settle and accumulate, restricting the flow of water out of the tank.
Some common symptoms of a slow draining water heater include: water draining slowly when you open the drain valve, water taking longer to fill up the tank, and reduced hot water capacity. To fix this, you'll need to flush the water heater to remove the sediment buildup. This is a task that many homeowners can do themselves, but if you're not comfortable with it, it's best to call in a professional plumber.