Garbage disposal black rubber "throat" is causing a slow-drain; why?

The black rubber "throat" in the garbage disposal sink drain is causing a slow-drain. Simply inserting a tool to open part of the rubber dramatically increases drain speed to normal. No other components are affecting drainage; just the rubber "throat". This is a new development; it used to drain fine. We have cleaned the black rubber part and have used Draino. The unit is 12 months old.