I've got 16GB (at 1866 MHz, DDR3), so that might have something to do with your performance. This game ate up around 6GB of ram, which was about 9-10GB of total system usage while playing.
As for performance savers, I found volumetric fog pretty heavy, extra streaming distance and object detail to be relatively impactful. I think all the shadow and spot shadow settings are probably more important though. I found something that worked for me and didn't fiddle too much with it (shadow quality high, spot shadows medium, spot shadow resolution high, contact shadows all high).
The following runs relatively well for me, still dips every now and then in very specific areas, but its' mostly 60.
https://imgur.com/a/lXF42E3
Like the other poster, you might be hurting for ram. What do you have object detail and extra streaming distance set to? Those seem to hit memory usage pretty hard.
Object detail appears to be the main setting controlling where LOD pop in takes place, higher being farther away. Not quite sure how it works in combination with streaming distance. Lowering streaming distance helped a lot when I was bouncing between 60 and 59 fps, causing stutter.