Located in C:\Users\(Your username here)\AppData\Local\Poostall_Royale\Saved\Config\Windows, once you've opened the file, copy and paste the following.

[SystemSettings]
r.MaxAnisotropy=16
r.SceneColorFringeQuality=1
r.DepthOfFieldQuality=0
r.Tonemapper.Sharpen=0
r.MotionBlurQuality=0
r.Streaming.PoolSize=2048 (If your GPU has more than 4 gig's of VRAM, set the number to half of your current VRAM.)
r.BloomQuality=1
r.LensFlareQuality=1
r.TemporalAACurrentFrameWeight=0.2
r.TemporalAASharpness=0.8
r.ScreenPercentage=80 (100 is the Medium resolution setting in the graphics menu, while 80 offers the best between high frame rates and visual quality. Experiment to your liking.)
r.TemporalAA.Upsampling=1
r.TemporalAA.Algorithm=1

Also located in the same folder as Engine.ini, once opened, just change sg.GlobalIlluminationQuality=3 to sg.GlobalIlluminationQuality=1.

Every other setting maxed out doesn't have the same performance impact as the global illumination, so just keep it on 1 or set it to 0, both options give a huge frame rate increase.

Right click the game in your Steam library, click on Properties, then on General, and type -d3d11 in the Launch options.
The game runs on DX12 by default, and downgrading to DX11 will increase the performance.

Now you're ready to s h i t all over NPC's like the Postal Dude or Postal Doe in high frame rate style.

Tested with a GTX 970, i7 6700k and 16 GB's of RAM.