Arma 3 loves fast processors, it does not care that much for amount of cores. It wants high clocked ones, the higher the clock speed is the better. What you can try is to set processor affinity to use all cores and also set the priority to high (NOT REALTIME). And Arma 3 is 32 bit and it really cant exceed that amount of ram. As for performance and low FPS. That is caused by your zombie mod. You see the server threads? Having 70 threads is WAY too much. The best value is around sub 20 threads. Even then 20 is sort of high. Most servers with a lot of players have around 5-10 threads max.
See my post here on information regarding what a thread is:
Also that spawn script you use for the zombies has triggers and that is REALLY bad for performance as well. So if you wish to run zombies this way you will either have to live with the bad performance or rewrite it because it's not really optimal at the moment.