and if you want good settings on a 2017 iMac : remember to set GameSettings.ini to readonly:
[ScalabilityGroups]
sg.ResolutionQuality=30.000000 ; Lower render resolution scaling
sg.ViewDistanceQuality=1 ; Minimal view distance
sg.AntiAliasingQuality=0 ; No AA
sg.ShadowQuality=0 ; Disabled
sg.GlobalIlluminationQuality=0 ; Disabled
sg.ReflectionQuality=0 ; Disabled
sg.PostProcessQuality=0 ; Disabled
sg.TextureQuality=0 ; Lowest textures
sg.EffectsQuality=0 ; Minimal effects
sg.FoliageQuality=0 ; Minimal foliage
sg.ShadingQuality=0 ; Lowest shading
sg.LandscapeQuality=0 ; Lowest landscape
[/Script/Engine.GameUserSettings]
bUseVSync=True
bUseDynamicResolution=False
ResolutionSizeX=1280 ; Lower base resolution
ResolutionSizeY=720
LastUserConfirmedResolutionSizeX=1280
LastUserConfirmedResolutionSizeY=720
WindowPosX=-1
WindowPosY=-1
FullscreenMode=0 ; Windowed Fullscreen (often better for iGPUs)
LastConfirmedFullscreenMode=0
PreferredFullscreenMode=0
Version=5
AudioQualityLevel=0
LastConfirmedAudioQualityLevel=0
FrameRateLimit=59.000000 ; Lock to 30 FPS for consistency
DesiredScreenWidth=1280
bUseDesiredScreenHeight=True
DesiredScreenHeight=720
LastUserConfirmedDesiredScreenWidth=1280
LastUserConfirmedDesiredScreenHeight=720
LastRecommendedScreenWidth=-1.000000
LastRecommendedScreenHeight=-1.000000
LastCPUBenchmarkResult=-1.000000
LastGPUBenchmarkResult=-1.000000
LastGPUBenchmarkMultiplier=1.000000
bUseHDRDisplayOutput=False
HDRDisplayOutputNits=1000