Here is the jist of it.
There is a bug that causes low settings to actually produce worse FPS than high settings. With the shadows disabled, you can actually play better on high settings than low.
1. Press ESC to get to your video settings (duhh)
2. Press HIGH
3. Go to Advanced tab and change the below settings.
a. Turn off all shadows (set to none)
b. Turn off water reflections (possibly, try it)
c. Might have to change from Shader 3.0 to 2.0
4. Press Apply then Ok
5. Enjoy your new settings.
*NOTE do not mess with ANY of the sliders
For me... I have been messing with Shaders 3 and 2. Sometimes 3 lags too much so I switch to 2.0. But before when I was running on the LOW vid settings I couldnt even try shader 3 it lagged so bad.
I think the reason this is working for some... is that like the OP said ... the default HIGH settings have some magical combination of what this game requires. Also I was reading the "shadows" is what might be causing a bunch of peoplese problems.
The thing about the sliders is this... once you move a slider your settings become special and not default. Even tho your defaulting to HIGH, it still runs better then when on LOW for some reason.
I played with this for a good amount of time yesterday and this morning and I really think it does help some to default to the HIGH settings then turning shader to 2 and shadows off. I was even playing with shader 3 this morning for like an hour and wow its never looked / ran so well!!!
Hope this helps some, it's worth a try.
Here is the link from the forums if you wanted to read up.
http://forums.ageofconan.com/showthread.php?t=36047
Here is what I'm runnin for comparison
AMD Dual core 3.2 gig cpu
XFX 7900GS oc'd with 256 ram
2 gig DDR2
running at 1440x900 with 30-65 fps steady