Cessna 208B pilot reported a rapid loss of airspeed during cruise resulting in a loss of altitude. The pilot regained control of the aircraft at the lower altitude due to ice shedding from the aircraft.

Narrative

After departure; climb; and level-off cleared direct ZZZZZ for the RNAV XX into ZZZ1 the aircraft's indicated airspeed fell rapidly from 135 KIAS to below 102 KIAS with an aggressive positive pitch to maintain altitude. Prior to the airspeed loss the aircraft was set for normal cruise power; inlet heat on; bleed air on; pitot/static heat on; and propeller auto deice mode on. Full power was applied immediately and Autopilot turned off to recover airspeed. De-ice boots were then applied again with indication and visual inflation of some boots observed. The engine indicated EGT above 650 with full power applied; however lower than normal fuel flow (PPH) was observed possibly indicating less than normal power was being produced; this lower fuel flow per power setting was also observed during normal cruise. Indicated airspeed showed 120 KIAS with the full power applied and indicated on the torque display. After the deice boots were applied airspeed stabilized around 125 KIAS with a continued aggressive positive pitch and an approximate 100-200 FPM decent. A strong vibration started and continued from the engine/propeller. Manual propeller deice was attempted with no change in vibration. Unable to maintain altitude and unable to maintain at least 130 KIAS with full power- I informed Center that I was 'unable to maintain altitude; had an engine vibration; and the engine was over temperature; I was immediately returning to ZZZ and planning to return via the RNAV XY via ZZZZZ1.' Center queried if I was [requesting priority handling] to which I replied Affirmative with number of souls and fuel onboard. Upon evaluating my decent; lack of power; and speed (1) I did not want to get further from ZZZ by heading to ZZZZZ1 (2) I did not want to go towards the 2;800 ft terrain feature at ZZZZZ1 and (3) knowing I would be below the crossing altitude at ZZZZZ1 but could make the altitude at the closer fix ZZZZZ2- I decided in the interest of safety and precaution to navigate direct to ZZZZZ2 and informed Center as such. Center informed me of the terrain to which I replied I was familiar and can maintain my own obstruction clearance. I met the crossing altitude at ZZZZZ2 and continued the remainder of the approach. Center provided IFR approach clearance once over ZZZZZ2. Once below 3;500 where temperatures warmed past -2C the vibration lessened then stopped and airframe ice noticeably (auditory/visually) began to shed from the aircraft.Maintenance discovered failed Propeller Deice boots upon landing; believed to have caused the strong abnormal vibration. Clear ice had formed pasted the protected areas. Possible TTL/Engine Temp Compensator malfunction may have prevented full power availability.