Build optimization, and texture cooking

I have followed some Epic tutorials to reduce my package size from 10GB to 2GB, still way to go, but little by little.
The cooker settings in project settings are one that confuse me, since I have no knowledge of the terms used.

PVRTC compression quality and speed? Does it mean cooking speed, or ingame performance? If its just the time spend on compiling, of course I will set it to best quality as it doesnt take much time anyway.
ASTC and the same issue.
Then there is ASTC size as well. How big of a difference does it do to file sizes/quality especially with high resolutions?