The settings discussed here can all be found in the GameUserSettings.ini file. That file can be located in your user appdata folder. You can copy and paste the below path into your windows run dialog if you want. Open the run dialog with [WIN]+[R].

Path to file: %userprofile%\AppData\Local\Maine\Saved\Config\WindowsNoEditor

I started with the Low setting and modified it to turn off all options (set to 0). Then I set one at a time to 1, then 2, then 3, restarting the game between each change.

The results are frames per second lost going from each setting level. For example from setting 0 to setting 1. If setting 0 I had 10 fps and setting 1 I had 8 fps, then the result would be 2. I indicate the setting changes like 0-1.
(lower is better)

For the ResolutionQuality setting, I had all other settings to 3 and my resolution set to 1920x1080. This is showing the frames gained by reducing the resolution scale.
(higher is better)

The testing results I got are not to be considered seriously. They were just my results on my system and it's configuration, your own results may vary.

• ShadowQuality was by far the largest impact, but only changing it from 0 to 1. From 1-3 the impact was nearly imperceptible in actual gameplay. You might be tempted to just turn this off then and save about 30fps, but don't, it looks like trash without any shadows. So for me it seems that if I'm going to have it on, I may as well set it to 3.
  
• I didn't notice any difference between the FoliageQuality and ShadingQuality settings, even completely turning them off seemed to have no effect, nor did any of the settings effect anything visually for me.

You can also use the following settings (paste them in) in your Engine.ini file. This has the effect of removing the Depth Of Field and Motion Blur settings. This is also useful if you don't like those settings anyway. It removes the blur on the distant objects and also removes any blur that happens when there is fast motion, like when you move quickly.
This increases my framerate by about 7fps.

• Pressing [SPACE] rapidly on startup helps bypass the intro logos quickly and also loads the first save.
  
• Press [ALT] + [F4] to quit the game much faster.

Low

Medium

High

Epic