TV lagging when watching regular TV channels while Apple TV is on
Apparently (as thought) the solution had nothing to do with the Apple TV but rather the TV itself being horribly built. 

In short, the reason is interference. If the HDMI cable and the TV cable were too close to each other the power/throughput from the HDMI cable would destroy the data coming through the TV cable. Yeah - what a genius build quality from Samsung.

The solution? Pick the HDMI port the furthest away from the TV cable and bend them in opposite directions and you're good to go. "What if I have several HDMI devices" you may ask? Your only solution seems to be to turn them off when you don't use them.