The primary design intent of speed boosters is exactly what the name suggests. They make the lens "faster" (i.e. provide more light to the image plane).
They are primarily an alternative to increasing ISO and secondarily an alternative to spending lots of money expensiver faster lenses. Being an alternative to higher ISO's sets the ...
The 35mm f1.8 DX lens is sharp as a pin, distortion-free and cheap as chips. If you want a 'normal' lens (eg for candids or small-group portraits) you can't do better. As others have said, no speedbooster made for Nikon F-mount bodies.
My question would be... why do you want a 50mm lens FoV? My opinion is that about the worst thing you can do in photography is to create "normal" images. And for many things the 75mm EFL is arguably better.
The speed boosters function by inserting additional optics into the light path... in my experience that always has some ...