If you are looking for a free solid CDN option for your child sites then one of the best options out there is Statically. Statically is powered by Fastly, Cloudflare, BunnyCDN, CDN77 which means it has a higher number of PoPs that will be serving the site assets.
Statically now has a plugin for WordPress which is very easy to setup. After you have installed and activated the Statically plugin you will need to go ahead and add your email address, site URL and then you will be able to generate an API key for the plugin.
The plugin settings for Statically can be found from;
Settings > Statically
Statically will be able to minify CSS and JS files on the fly as well optimize images. It also means that all site assets will be served from Statically CDN. On the fly, CSS and JS minification along with image optimization is enabled by default in the plugin settings for Statically.
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Using Statically CDN will improve site assets (CSS and JS) as well as image asset load times with those being in a number of cases most of a sites size, it will have the knock on effect of improving your sites load times easily.