Its an open world game, and the scale of it makes it CPU hungry, its also has some fairly good physics which need to be constantly applied to the game, through out the open world. I run it fairly well, maxed out settings I get about 38fps.
It also has to be said the engine running the game isnt too badly optimised but can do with a few improvements.
Killzone shadow may look better but its not trying to render a full open world with a wide viewing distance, its a small scaled game, arcade like therefor it doesnt have as many intense or realistic core mechanics as arma 3, and this obviously puts the load off the cpu