Pasting this here for future reference (from an email generated by WPMU Dev):
Site speed isn’t all about images though – there are heaps of other things to look at, too. I’ve collected a few posts from our blog that nicely sum up what you should look at to get your site operating at 100% optimal speed.
Here’s a quick overview of five areas to work on
This one is a comparison of a few top caching plugins (WP Super Cache, W3 Total Cache and WP Rocket)
A simple list of the 10 fastest WordPress themes
You like image optimization, so here’s some more detail about that
This is a big 22 step outline of every aspect of optimizing speed on your wordpress site
And finally, an in-depth drill-down on how we managed to get a WordPress site to a 91/100 on Google Pagespeed Insights