Less blogposts here lately, mostly because I’m doing custom Autoptimize-development for a partner (more on that later) and because I get a lot of support-questions on the wordpress.org support forums (with approx. between 1500-2000 downloads/ weekday that is to be expected). One of the more interesting questions I got there was about Autoptimize being slow when JS optimization was active and what would be the cause of that. The reply is of interest for a larger audience and is equally valid for CSS optimization;
Typically the majority of time spent in Autoptimize is mainly in the actual minification of code that is not minified yet (purely based on filename; if the filename ends in .min.js or -min.js).
So generally speaking, the way to avoid this is;
1. have a page cache to avoid requests triggering autoptimize (as in that case the cached HTML will have links to cached CSS/JS in it)
2. for uncached pages; make sure AO can re-use previously cached CSS/ JS (from another page), in which case no minification needs to be done (for that you will almost always want to NOT aggregate inline JS, as this almost always busts the cache)
3. for uncached CSS/ JS; make sure any minified file is recognizable as such in the filename (e.g. .min.css -min.js), this can lighten the minification-load considerably (I’ll add a filter in the next version of AO so you can tell AO a file is minified even if it does not have that in the name).
So based on this, some tips;
* make sure you’re not aggregating inline JS
* for your own code (CSS/ JS); make sure it is minified and that the filename confirms this. if you can convince the theme’s developer to do so, all the better (esp. the already minified but big wp-content/themes/bridge/js/plugins.js is a waste of precious resources)
* you could try switching to the legacy minifiers (see FAQ) to see if this improves performance
* you can also check if excluding some un-minified files from minification helps performance (e.g. that bridge/js/plugins.js)