I used to have performance issues with Dayz SA on my mid-range i5 quad-core machine with a decent graphics card (NVidia GTX 760), even with modest graphics settings. FPS dropped to 18 in big cities and I was not the only one complaining about this. If your machine suffers from this, monitor your CPU temperature. CoreTemp and CPU-Z are good options. Turns out I had a whimpy standard cooler (boxed), probably even incorrectly mounted. Whenever DayZ stressed the CPU (70% load on most cores), temperature went up to 100°C and clock frequency went down automatically to prevent the thing from being burned alive, reducing Dayz performance to no-fun-at-all. I wouldn't be surprised if there were more experts like me out there with crappy CPU cooling, never noticing anything until they play DayZ and their quad-core is really challenged for the first time :O)