evening Mike,
here are some tips:
* install a page cache (you seem to be on nginx, which can do page caching, but I only see cache misses)
* you have something inserting a significant amount of minified CSS for your mega-menu in your site with a date/timestamp in it;
<style type="text/css">/** Wednesday 27th June 2018 14:47:18 UTC (core) **/#mega-menu-wrap-menu-header [...]
if that timestamp changes regularly or if it is different between different pages, it would be a perf-killer as it would bust AO’s cache and force the CSS to be re-minified
* this might happen with other inline CSS or JS; keep on eye on your cache size, if you see the amount of files in wp-content/cache/autoptimize/js or wp-content/cache/autoptimize/css go up quickly, then this probably means your cache hit ratio is too low due to changing inline CSS or JS, meaning you have to (re-)configure AO to exclude those from optimization.
* another thing (but only if you developed some stuff yourself); try pre-minifying your CSS & JS and make sure the filename has min. in it, e.g. jquery-migrate.min.js or mystyle-min.css, as that way AO knows that those don’t have to be re-minified
hope this helps,
frank
