Re: HTTP/2 JS and CSS aggregation
I did some tests with Lighthouse and it seems to suggest that leaving aggregation on is faster.
My thinking was with HTTP/2 you don't need to aggregate but I guess if the server and the browser are still dealing with 30 odd JS and CSS files it will slow things down.
Maybe outside of the test environment it would be quicker as the individual files are cached and can be used on any subsequent pages. As it is now.. sometimes the big aggregated files can be used on a subsequent page and other times it's another big aggregated file that has to be downloaded.
Re images.. when I design a site I use 'View Image Info' in the right click menu in Firefox to find out the actual size my images are being displayed at and then set the image creation script to make them that size.
There's an image CDN that can serve the correct image size based on the container size at the current pixel width of the page. I think mod_pagespeed does this as well. We are waiting for our host to implement this.
Core version: 5.3.xx
Web server: Apache