In reply to Diogene.

I agree that in general you should be careful when using CDN, even for this plugin I recommended testing and wrote FAQ entry “Will my site be faster when I start using commonWP?”.

However, note that there is one difference between this and other CDN solutions. When using commercial CDN, each file is used only by your own site, not by others. That means that your visitor almost certainly won’t have it cached in their browser, and that unless you have large traffic from all over the world, it won’t be cached even in some POPs of CDN.

With this solution, you are offloading only files that are used by open source plugins, themes, or WordPress core itself, and that are used by many other sites (in WordPress core case, we are talking about over 30% of sites). Some files are used on almost all WordPress sites (jquery.js, jquery-migrate.min.js, wp-embed.min.js, wp-emoji-release.min.js). Even now, with just around hundred of sites that are using commonWP, those files are mostly cached by CDN’s POPs around the world, I have tested that (and anyone can too with switching locations in VPN/TOR and watching response headers). With more sites using it, we are increasing chance that this is true for files from popular plugins as well (Jetpack, WooCommerce, Gutenberg…), and chances are increasing that some files might already be cached by visitor’s browser. More sites = better results. I wrote this in FAQ entry “Are there any benefits other than speed?“.

Also note that you can tell commonWP to offload only some files to CDN, for example only those popular files. There is a code example under “Deciding should path be rewritten“.