Polyline inherits from Rhino.Collections.Point3dList and PolylineCurve inherits from Rhino.Geometry.Curve. Meaning that they expose different properties/methods from their parents. Which is faster would depend on the use case. Hope you’re doing well 
Polyline inherits from Rhino.Collections.Point3dList and PolylineCurve inherits from Rhino.Geometry.Curve. Meaning that they expose different properties/methods from their parents. Which is faster would depend on the use case. Hope you’re doing well