DCS only uses 2 cores. One for the game one for the sound. You need to check individual core useage not overall CPU useage.
Also can you check you GPU load? I'm pretty sure you will discover a bottleneck... Either cpu core pegged at 100% or the GPU.
DCS currently doesn't much like AMD cpus.. at least older ones. I gained 20fps going from an old Phenom 2 at 4.2ghz to an i7 at 3.9!