Your graphics card is the bottleneck. I believe FFAA stands for Feature-Following Anti-Aliasing. Although in Arma I think you have FSAA (Full Screen Anti-Aliasing).
Basically when textures are viewed at an angle or at a distance, they don't fully 'fit' your screen. So some post-processing is performed to make them look smooth instead of grainy.
See the images under Examples in this page:
https://en.wikipedia.org/wiki/Spatial_anti-aliasing#Examples
I imagine this comes forward on Sangin with the numerous vegetation. Each bush will need some of this anti-aliasing along the edges.
Short story: Graphics card too old.