ArmA 3 is CPU limited, so graphics settings won't unduly influence performance.
Try bringing your draw distance down to about 1000m in game--and this setting won't save for me for some reason, so you may have to change it each time you load in to a server. That will reduce the strain on the CPU. I gain about 10FPS doing this.
You can also try running other games to see if performance is lower than expected. If other games run fine, then you've got a case of ArmA being ArmA.