The answer to 'backflow preventer slow draining' is that a slow-draining backflow preventer is usually caused by a clogged or malfunctioning device. This can happen over time as debris and mineral buildup accumulate, preventing proper water flow. Symptoms include slow draining sinks, tubs, or other fixtures connected to the backflow preventer.
The solution often involves disassembling and cleaning the backflow preventer. This may require specialized tools and knowledge, so it's generally best to call a professional plumber for this task. They can inspect the unit, identify the root cause, and make any necessary repairs or replacements to restore proper drainage.