n00b needs help: few performance diff. between low and high

[palvas]

My specs are in signature: usually i make 40 fps on the channel and 26 on ground,with stuttering ecc.
With settings in VERY LOW ,shadows,trees and buildings off,i don't notice any important difference: 50 on water and 34-30 on ground,stuttering always present (sometimes less,sometimes more. FPS number very confusing: it goes from 30 to 11). It's not a complaining/rant.
I tried everything (ini,epilepsy filter thing,aero thing ecc.). People and friends with worse pc than mine make 70 fps on water on high.
I have the impression that i've missed something ? PC specs too low ?