HI,
preload and delivering critical Js/Css code doesn't speed up your web site in most of the cases.
It gaves you the illusion that things will be faster, but, in a few occasion it could became worst.
You can check this link: https://developer.mozilla.org/en-US/docs/Web/HTML/Link_types/preload
But IMO you can speed up your site doing these things:
- using appropriate image compression (avoid .png images and any large images)
- using a mix of colours-B/N images (lighter)
- disabling unused modules (desktop AND/OR mobile)
- disabling the preview animation product at least inside home page
- avoid using too much (and heavy) sliders/animations
- avoid using too much categories and themes that trigger dozens of requests on the server
- avoid using duplicated content/modules (generally happens with facebook buttons, multiple chat systems etc...)
- using Apache Gzip compression AND http2 on server
- using cache appropriately
- checking php.ini file (and options) through cpanel
- optimizing MySql db through phpmyadmin (just the automatic process is ok)
- reducing web crawlers/bots visiting your site through robots.txt and .htaccess
Danny