these were the modifications i've used when i tried to mod the game using dnSPY then tested on a potato pc and everything was smooth:
Screen.SetResolution(1366, 768, Screen.fullScreen);
QualitySettings.globalTextureMipmapLimit = 4;
QualitySettings.masterTextureLimit = 4;
QualitySettings.anisotropicFiltering = AnisotropicFiltering.Disable;
QualitySettings.antiAliasing = 0;
QualitySettings.lodBias = 0f;
QualitySettings.particleRaycastBudget = 0;
QualitySettings.pixelLightCount = 0;
QualitySettings.realtimeReflectionProbes = false;
QualitySettings.resolutionScalingFixedDPIFactor = 0.5f;
QualitySettings.shadowDistance = 0f;
QualitySettings.shadows = ShadowQuality.Disable;
QualitySettings.softParticles = false;
QualitySettings.softVegetation = false;
QualitySettings.streamingMipmapsActive = false;
QualitySettings.vSyncCount = 1;
dnSPY path to method (in anycase someone might want to mod while waiting for graphics settings);
Assembly-CSharp -> Assembly-CSharp.dll -> Management -> GameSettingsSystem -> LoadAndApplyGraphicSettings()
tool used: https://github.com/dnSpy/dnSpy