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 or mineral deposits in the tank. This can restrict the flow of water out of the tank, leading to slow drainage. Symptoms of this issue include slow draining, difficulty draining the tank completely, and decreased hot water capacity.
To fix a slow draining water heater, you'll likely need to flush the tank to remove the sediment buildup. This is a relatively simple DIY task that most homeowners can do themselves. However, if you're not comfortable doing it or the problem persists, it's best to call in a professional plumber to inspect and service the water heater.