Manual:$wgImagePreconnect

Details
Add a preconnect link for browsers to a remote FileRepo host.

This is an optional performance enhancement designed for wiki farm where or  is set to serve thumbnails from a separate hostname (e.g. not local " /w/images "). The feature expects at most a single remote hostname to be used.

If multiple foreign repos are registered that serve images from different hostnames, only the first will be preconnected.

This may cause unneeded HTTP connections in browsers on wikis where a foreign repo is enabled but where a local repo is more commonly used.