Candy's right - a good cyclist can ride off on a crock of **** at 25 mph. For a short time, anyway, as they will throw it in a ditch because it's so uncomfortable.
You have to put it all in context. There's no point in going for ultra lightweight if you are a biffa. I have promised myself that until I lose half a stone, I will never buy a carbon fibre bottle and bottle holder.
It's a bit of a misconception that steel bikes are heavy - pair them with a lightweight groupset, pedals, seat pin, wheels etc. etc. and they can get pretty light. If you can be bothered.