The connection image quality is very related with the application performance (higher quality=lower performance).
The default Image quality is Optimum, because it presents the best cost benefit relationship between quality and performance. If you need to have more quality or better performance, take a look at the other options below:
Highest - Uses PNG images only (0% compression)
Optimum - Combines PNG and JPEG images (20% compression).
Good - Uses JPEG images only (40% compression)
Fastest - Uses JPEG images only (50% compression).