First you gotta go to steam library, right click skara, press properties, local files and click on view local files.
Then you gotta head to SkaraGame\Config, open defaultgameusersettings and do this:
For max performance:

Originally posted by author:

bUseVSync=False
bUseDekstopResolution=False
Volume/music volume matters on your personal prefference.
Graphicsquality=1

Max graphics:

Originally posted by author:

bUseDekstopResolution=true
Graphicsquality=3

Then head to

Originally posted by author:

SkaraGame\Saved\Config\WindowsNoEditor , open GameUserSettings

and do this:
For max performance:

Originally posted by author:

GraphicsQuality=1
Fullscreenmode=personal prefference
Resolution is based on personal prefference/possibility, if you are really laggy you can use 640x480 or even lower
bUseDekstopResolutionForFulscreen=based on personal prefference/possibility again.

For max graphics same,but you gotta make GraphicsQuality=3

And then, you gotta change those:
Max performance:

Originally posted by author:

sg.ResolutionQuality=100, lower it if you want the game to look like gta IV on ultra low settings
sg.ViewDistanceQuality=1
sg.AntiAliasingQuality=1
sg.ShadowQuality=1
sg.PostProcessQuality=1
sg.TextureQuality=1
sg.EffectsQuality=1

For maxed graphics:

Originally posted by author:

sg.ResolutionQuality=100
sg.ViewDistanceQuality=3
sg.AntiAliasingQuality=3 note this and other one can change fps by a lot, only use if you really have a beast pc.
sg.ShadowQuality=3 and this, both can change fps a lot, only do if you really have a beast pc.
sg.PostProcessQuality=3
sg.TextureQuality=3
sg.EffectsQuality=3