facetime on apple tv

You can use AirPlay with FaceTime on the Apple TV, if that's what you mean.

Even if you can, doing so is un-natural. You're not looking at the camera anymore, and there's additional delay by the necessary processing to stream it to the display. It's best to stick with the device itself for Facetime I dare say.

There is no additional delay, if that's what you are experiencing then you have a problem with your network. We use FaceTime on the TV in the kitchen via the iPad with our son in university while we eat and have no such problems. The iPad sits on the small shelf below the wall mounted TV, so everyone looks at the camera.