Wikibase\SitesModule is a content-versioned module, and in this profile of a startup module request, 16.5% of the time is spent in Wikibase\SitesModule::getScript(). This function caches its output, but only for 10 minutes, and it uses the server cache. Given its performance impact, we may want to consider caching it in WANCache instead, and for a longer period of time (e.g. 24 hours).
If there's way for this module to provide a definition summary that is guaranteed to change when the module contents change, but cheaper to compute, that would also be helpful. But I doubt that is the case given the fact that 2/3 of its time is spent in DBSiteStore::getSites(). The module also does message parsing, but that doesn't appear to contribute much to its slowness (probably because it only does it for a few "special" sites).