If you haven't paid too much attention to the development of the game, one of the largest changes they've put in is reworking the networking on the engine to implement a 'network bubble'. What that does is only provides players with information of things that are within X meters of them (I think he said 1400?). The way it works in the ARMA series is every player on the map has all the information on the map, where every player/zombie/item/ect is. This prevents people from running scripts to teleport everyone into the ocean, ect.
The zombie AI is also ran on the server instead of the client (arma).
Both of those features will boost client FPS. Because they have access to the engine, they've been able to transfer performance heavy functions over into it making them perform up to a 100x quicker.