spacedesk uses image compression to improve display performance. 

This mechanism reduces image size before transferring images over the network. 

The spacedesk image compression algorithm is lossy. It reduces quality and color depth of the image.
On fast networks, color depth and image quality can be kept very high while still achieving good performance.
On slow networks, color depth and image quality need to be reduced to achieve good performance.

The “Compression Quality” dialog box of the Windows VIEWER Settings allows the user to enable or disable the compression of screen updates. 

To choose OFF means to use RGBA 32-bit color format of screen image and no image quality reduction will be done. 

To choose On means that the user allows to reduce color depth by Chroma Downsampling and adjusts the compression quality to find the ideal image quality and performance speed.

YUV 4:4:4 
No reduction of color information. High quality of screen image but, performance intensive. Can only be used on very fast networks.

YUV 4:2:2
Reducing the color information by 1/3 (from 24 bit to 16bit) reducing color quality of screen image and improving performance.

YUV 4:2:0
Reducing the color information by 50% (from 24 bit to 12bit) to achieve best possible performance.

The color loss caused by the above mechanism typically not very much noticeable for the human eye, especially when there is a lot of motion on the screen (e.g. playing a video). Thus YUV: 4:2:0 is the default setting which can apply to many user Applications.
The Image Quality can be varied between 100 (no compression) and 0 (maximum possible compression). 
The default value is 70 which still tends to show a clear enough and detailed enough image for many user applications.