The aircraft's rate of climb deteriorated in icing conditions, while the aircraft was experiencing mountain wave effect during the latter part of a climb to FL130. Once level, the aircraft did not accelerate as expected and a descent was initiated, with the autopilot engaged using vertical speed mode. An unusual vibration was then experienced, followed by a stall warning system activation and the autopilot disengagement. The aircraft was recovered in a non-standard manner, a safe airspeed was achieved and the autopilot was re-engaged. Subsequently, a normal landing was made at the planned destination. Unusual ice formation, seen on the wings, corroborated calculations by the manufacturer that the aircraft had encountered severe icing.