If you have a low spec system - laptop? the biggest problem will be file access and ram/vram. If you don't have this on an ssd, defrag your drives, it'll lower the access times of files to improve performance by a percent or two - shaders and textures are the biggest hits.

Lower the resolution you play at, this will reduce the ram/vram usage as well as the cpu/gpu requirements. The only graphics settings that matter are LOD (level of detail), anti-aliasing, shadows, and anisotropic filtering, turn those all to their lowest or off. Most other settings don't do anything performance wise.

The FPS drops when you move are because of shader and texture loading.