Try these settings:
[API.Hook]
LastKnown=64
d3d11=false
d3d12=true
d3d9=false
d3d9ex=false
OpenGL=false
Vulkan=false
[Window.System]
Borderless=true
Center=true
RenderInBackground=true
Fullscreen=true
OverrideRes=3840x2160
[Render.FrameRate]
TargetFPS=50.0
SleeplessRenderThread=true
SleeplessWindowThread=true
PreRenderLimit=3
BackBufferCount=2
PresentationInterval=-1
[SpecialK.System]
EnableCEGUI=false
ShowEULA=false
[Display.Output]
ForceWindowed=true
[Render.DXGI]
UseFlipDiscard=true
AllowTearingInDWM=true
SwapChainWait=10
Adjust your TargetFPS accordingly, as well as OverrideRes.
I have been using these in Borderless mode and the results are quite favorable.