The nagant is currently very underpowered compared to every other pistol, because is the only single action side arm in the game, making it harder to take out a target as fast as any other pistol, while it is true some single action nagants where used during WW2, most nagants used where double action, due to the fact that the red Army only issued double action ones, with the single action revolvers been a surplus gun that came from the captured weapon stocks of the zarist white army.
As such there is both a gameplay reason and a historical/immersion one to make the revolver double action.
Yeah I noticed the fire rate from the hip, but losing the sight picture makes it harder to take out enemies at anything but point blank range spam. I guess that the double action animations would fix that.