The answer to 'why is my sump pump slow draining' is that the pump may be clogged, the discharge line may be blocked, or the pump may be failing. A slow draining sump pump can be caused by debris or sediment buildup in the pump itself, a kink or blockage in the discharge pipe, or a problem with the pump motor or impeller.
If the sump pump is slow to drain, first check the pump inlet and discharge line for any obstructions. Remove the pump and clean out any debris or sediment. Also inspect the discharge line for any kinks or clogs. If the pump still seems to be running slowly, the motor or impeller may need to be replaced. In some cases, the entire sump pump may need to be upgraded or replaced if it is old or undersized for the application.