ok so, i have tested a bit, and my conclution is this:
Rendered image with 0% Compression took me 0:53sec and File size: 3000Kb
Rendered image with 100% Compression took me 0:56sec and File size: 365Kb
they look just the same…
so is there ANY reason NOT to just flip the compression to 100%? the rendering time is barely noticable, but the filesize difference is MASSIVE!
im using PNG format, maybe it matters with other formats? i dont know.