Basically, each and every WordPress theme will definitely make your website looks nice and functional, but in order to improve performance, you’ll need a tool or plugin that will minify CSS and JS files, combine them and move them above the fold so that your website passes Google PageSpeed test with higher score.
Fast Velocity Minify is the plugin that will offer you the ability to make your website faster by minimizing the size of all CSS and JS files on your WordPress website, combine all those separate HTTP requests into one combined file. The plugin also offers numerous options for adjusting the level of caching and to select the areas you want to optimize.
The plugin focused on true needs of optimization, so you can cache and minify pages and posts, you can find many useful tweaking options inside that you won’t find with any other caching plugins like W3 Total cache or similar in the free version.
This plugin is not real substitute for other caching plugins, its more like addon for managing minification better.
Fast Velocity Minit plugin is also compatible with many popular caching plugins and it offers cache auto purge support on W3 Total Cache, WP Supercache, WP Rocket, Wp Fastest Cache, Cachify, Comet Cache, Zen Cache, LiteSpeed Cache, Nginx Cache (by Till Krüss ), SG Optimizer, Godaddy Managed WordPress Hosting and WP Engine and more.
One of the key things worth mentioning that this plugin offers the option to keep the order to CSS and JS files while they are combined and minified. It helps in compatibility with plugins and themes for sure.
You can download this plugin and you can read more about it on the official Fast Velocity Minify plugin page.