Until the implementation of HTTP / 2 in our servers, many strategies have been implemented to allow the download of the contents of our site were as fast and effective as possible, including the one called domain sharding .
This common technique, used in HTTP / 1.1, basically consisted in parallelizing content loading using subdomains of the main domain pointing to the same server, or mount the same resources on different servers, with that we tricked the browser to make content downloads in parallel, open more TCP connections, instead of sequentially, reducing the load time of a page. A very interesting improvement to optimize our page .
HTTP / 2 eliminates this need for parallelization because you can request as many resources as you need on the same connection. In fact it is likely that, using domain sharding performance will be detrimental since it creates additional TCP connections and makes it difficult for HTTP / 2 to prioritize resources, in addition to performing DNS actions unnecessary.
Note: it is difficult to measure the perjury since it depends on the amount of resources requested per domain and the weight of each of them.
So if you have implemented a widget that enables domain sharding or you have made changes to your theme to implement this technique, I advise you to disable them and use only the main domain over SSL via HTTP / 2. To undo the change, also remember that all sub-domains used, continue redirecting to the main domain, so as not to lose old links to content.
If still, you decide to maintain domain sharding, remember to make sure that your SSL certificate of domain has a wildcard that makes it valid for all domain names used, or that you have an appropriate multi-domain certificate. This will keep domain sharding and HTTP / 2, and will prevent the browser from unnecessary errors from mixed content (HTTP and HTTPS content over a secure page).
And for all those that you have not yet implemented HTTP / 2 on your servers, I remind you that it is free and very easy provided that your hosting provider integrates Let's Encrypt + HTTP / 2 in its service offer.