Honestly, I've not much knowledge about current MF lenses, weight and speed. But I found a Olympus Zuiko for E-system: 150/2 weight 1.35 kg. Make that an MF lens and you'll probaly end up with 300 grams more as well? I really don't know, BC
On the other hand: Nikon could do a lighter version which also delivers nice bokeh.
On the other hand: Nikon could do a lighter version which also delivers nice bokeh.