When generating the startup response for a wiki, ResourceLoader needs to compute the content and version of the mediawiki.jqueryMsg module. This currently involves invoking the Parser.
The Parser adds performance overhead that I believe may've been unintentional.
It is coming from WikibaseClient's ResourceLoaderJqueryMsgModuleMagicWords hook.
Flame graph:
# function doResourceLoaderJqueryMsgModuleMagicWords { $magicWords['WBREPONAME'] = $this->getRepoName( $lang ); # function getRepoName { $message = new Message( $repoSiteName ); $message->inLanguage( $lang ); if ( $message->exists() ) { return $message->parse();