Just to give you some idea what weight does for speed on a bike.
10 or so years ago I went from a 45 lb bike to a 23 lb bike. My average speed didn't increase even a full mile per hour. And I was already in decent riding shape. The less weight did let me go for longer rides without tiring as quick. And I could climb a hill much more effortlessly.
In those ten years I've increased my average speed by a good bit, but only from riding more. Not by changing things on my bike. Bike changes only help if you have to be the first one across the line in a sprint or first to the top of a hill. And if you don't put in the miles per week to build up the speed you can average for an entire ride, then you will be too late to the finish line to have that sprint with your light bike because the others will already be finished with their ride.