I love this game, but just like every game on the engine, it takes a year to kill anything, even with absurdly overpowered weapons.
I've got a standard NCR trooper and an Assault Rifle. I was consistently getting headshots. It took 5 shots to get his head crippled, and I ended up running out of ammo and finishing him off with a power fist.
NV is supposed to be harder, but this is just unrealistic. I should be able to get a headshot with something like an assault rifle that instantly kills a normal human without a helmet.