VNC Viewer Compression

Menu path: Setup > Sessions > VNC Viewer > VNC Viewer Sessions > [Session Name] > Compression

Compression level (default = 2): Allows you to select the compression level; 9 is the highest compression (default: 2)
JPEG quality level: Allows you to select the image quality. 1 means the highest compression and the lowest image quality, 9 means the lowest compression and the highest image quality. (default: 8)