The frame and the wheels are the two heaviest parts of a bike so it makes sense that those are the first two places to make changes to reduce weight. Swapping out the chain, changing over to a cassette instead of a freewheel, and swapping saddles makes a good next pass.

A derailleur that's considered very light would be under 200 g. A heavy one is twice that, but still only a 200 g difference. 5x as much weight savings can be easily made with a wheel swap from cheap clinchers -> nice tubulars.

Without getting into components so light as to border on unsafe I would guess around 17 lbs is the lower limit for a C&V weight-weenie bike. There are others on this board who have attempted such a feat and can chime in with actual numbers.