I was looking at http://marcjschmidt.de/blog/2013/10/25/php-imagepng-performance-slow.html and towards the end of the document, I learned something from the chart at the end.
If I use a very high compression on an image, the processing time (including TTFB) will be higher and the download time will be smaller.
The opposite is true if I use low compression.
The problem here is if I pick too low of compression, then it will take substantially longer for people to load images on my site. If I pick too high of compression, then I believe the TTFB (time to first byte) value will be high. If that value is too high, then google might see my image loading speed as slow.
Currently for the desktop version of my site, I'm using full-sized images at 80% JPEG quality, and for mobile site, I'm using 66% quality.
I'm afraid if I raise the quality of the image (aka reduce compression) on either site, then there will be more data to download and some people might be billed high for data usage. My other option is to adjust the image size some more, but then if I go too small, visitors might complain.
So whats the best thing to do? adjust JPEG image quality to different values (more optimal values)? or do I shrink the images and pray for no complaints?