built qualilty: noisier motors, shorter lifespan, coating, less built like a tank etc.
performance: optical quality, sharpness, CA, AF speed, accuracy etc.
but depends on which lens. there are exceptions to the general observation.
i don't mind lighter plastic mount lens, as i don't go photographing war conditions, and don't drop stuff carelessly.
some 3rd party have useful ranges 1st party don't have. and easier on the pocket.