Well... the reason I asked is that all else equal I would expect a windscreen to slow the bike down not speed it up.
There is a sweet spot for windscreens where they present a more aerodynamic shape than your torso, and they effectively toss the air around your body. A little bit off in any direction and you are just pushing the air out of your way twice.
Big cruisers can compensate for this no issue and as a result they can just slap a nearly flat hunk of plexi on top of their forks. Lowers fatigue and bug splatter on your latest Sturgis t-shirt, happy rider. You would probably drop 5mph if you tried the same windscreen on a 125.
Dropped a gear... . . . . . . . . . . ...still here