The answer to 'why is my backflow preventer slow draining' is that a clogged or faulty backflow preventer is the most common cause. Over time, debris and sediment can build up in the backflow preventer, restricting water flow and causing slow drainage. This is a common issue that often requires professional service to properly diagnose and repair.
Other potential causes include a blockage in the main drain line, a problem with the sewer system, or an issue with the plumbing fixtures connected to the backflow preventer. If the slow drainage is isolated to a single fixture, the issue may be localized to that fixture rather than the backflow preventer itself.