Ok, figured out my CPU scaling thing in GNU/Linux. I'm stupid; had it set to "power saver" mode! In case it helps anyone else:
To check CPU mode:
cpupower frequency-info
To set to "performance" profile:
sudo cpupower frequency-set -g performance
To set back to "power saver":
sudo cpupower frequency-set -g powersave
This revelation got me another 5-10 fps.
Lowest it ever drops now is 28, generally hovers around 35 fps. This is with AA off (the AA looks really weird in this game anyway), textures, foliage, and shadows set to "medium", view distance bar set to the minimum, and at 1920x1080, full screen.
To me, game looks nearly the same as it does with everything maxed, and I get way better framerates. It's a good compromise between visual fidelity and performance.
Haswell i5-4670k, 8 gig of RAM, GTX 660 (2 gig of VRAM), Manjaro Linux