One of the major problems with Arma 3 is that due to it's simulation nature, it has to simulate everything, rather than just a bubble around the player and then

One of the major problems with Arma 3 is that due to it's simulation nature, it has to simulate everything, rather than just a bubble around the player and then cutting corners everywhere else. This means it inherently must use a whole lot more CPU than your average videogame. It's quite possibly badly optimised too, but even if it weren't, Arma would eat CPU like crazy.