There are over whelming evidence that fairing(in this case, called windshield) can do for better aerodynamics. You can search by typing "fairing bicycle" in Google, you get all kind of informaiotn,

Just a few expample.
Just about all racing motorcycle use fairing. If a naked (no fairing) motorcycle has better aerodynamics, they would not use fairing. At more than 200 mph, aerodynamics is one of the biggest concern.

At the opposite (slow)end, when people design a very energy efficient vehicle, such as man-powered or solar-powered for record setting purpose, one of the first thing they design is the fairing.

So, why bicycle racers are not using it(specially in time trial)? Because it's prohibited by UCI rule.