The game isn't optimized all that well to be honest, but I'm sure you already know that. It might be that it doesn't really spread across multiple cores that well. I'm sure that newer quad core i5 would run it much better than that Phenom II.
My best advice is to look up some DayZ optimization guides. I know that some of the settings in the game can really drag down performance. I don't remember which ones off hand, but you for sure don't want some of them enabled. Another interesting thing is that some settings you should turn up, so that it shifts the load to the GPU instead of the CPU. I think shadows are supposed to be turned up, because if they are on low it makes the CPU do the work which means less performance. Video Memory should also be on Default, otherwise it won't use VRAM correctly.
So do some googling and try to find an optimization guide. I can look for one later today too once I get home and see if I can find a good one.