I just noticed this huge blob being included on every page view of en.wikipedia.org, test2.wikipedia.org and presumably lots of other wikis as well. This wasn't there not so long ago, so either the extension regressed or it wasn't installed or enabled in this way before.
This is will likely impact page render time due to this configuration being inside the <head> and thus slows down retrieval of more important things (like stylesheets and article content). With the goal of trying to squeeze the first stylesheet and above-the-fold content into 12KB, we can't give 5KB of that just for CodeMirror configuration. (T127328)
This configuration should be removed from the default output and moved to a DataModule. Given VisualEditor supports a plugin list with module names, this shouldn't pose any problem.
5659 bytes
"extCodeMirrorConfig":{"pluginModules":[],"tagModes":{"ref":"text/mediawiki","pre":"mw-tag-pre","nowiki":"mw-tag-nowiki"},"tags":{"pre":true,"nowiki":true,"gallery":true,"indicator":true,"timeline":true,"hiero":true,"charinsert":true,"ref":true,"references":true,"inputbox":true,"imagemap":true,"source":true,"syntaxhighlight":true,"poem":true,"section":true,"score":true,"templatedata":true,"math":true,"ce":true,"chem":true,"graph":true,"maplink":true,"categorytree":true},"doubleUnderscore":[{"__notoc__":"notoc","__nogallery__":"nogallery","__forcetoc__":"forcetoc","__toc__":"toc","__noeditsection__":"noeditsection","__notitleconvert__":"notitleconvert","__notc__":"notitleconvert","__nocontentconvert__":"nocontentconvert","__nocc__":"nocontentconvert"},{"__NEWSECTIONLINK__":"newsectionlink","__NONEWSECTIONLINK__":"nonewsectionlink","__HIDDENCAT__":"hiddencat","__INDEX__":"index","__NOINDEX__":"noindex","__STATICREDIRECT__":"staticredirect","__NOGLOBAL__":"noglobal","__DISAMBIG__":"disambiguation"}],"functionSynonyms":[{"ns":"ns","nse":"nse","urlencode":"urlencode","lcfirst":"lcfirst","ucfirst":"ucfirst","lc":"lc","uc":"uc","localurl":"localurl","localurle":"localurle","fullurl":"fullurl","fullurle":"fullurle","canonicalurl":"canonicalurl","canonicalurle":"canonicalurle","formatnum":"formatnum","grammar":"grammar","gender":"gender","plural":"plural","bidi":"bidi","#language":"language","padleft":"padleft","padright":"padright","anchorencode":"anchorencode","filepath":"filepath","pageid":"pageid","int":"int","#special":"special","#speciale":"speciale","#tag":"tag","#formatdate":"formatdate","#dateformat":"formatdate","noexternallanglinks":"noexternallanglinks","#property":"property","#statements":"statements","#if":"if","#ifeq":"ifeq","#switch":"switch","#ifexist":"ifexist","#ifexpr":"ifexpr","#iferror":"iferror","#time":"time","#timel":"timel","#expr":"expr","#rel2abs":"rel2abs","#titleparts":"titleparts","#lst":"lst","#section":"lst","#lstx":"lstx","#section-x":"lstx","#lsth":"lsth","#section-h":"lsth","#target":"target","#babel":"babel","#coordinates":"coordinates","#invoke":"invoke","#related":"related","#assessment":"assessment","#categorytree":"categorytree","#pagesusingpendingchanges":"pagesusingpendingchanges","pendingchangelevel":"pendingchangelevel","articlepath":"articlepath","server":"server","servername":"servername","scriptpath":"scriptpath","stylepath":"stylepath","wbreponame":"wbreponame","numberofwikis":"numberofwikis"},{"NUMBEROFPAGES":"numberofpages","NUMBEROFUSERS":"numberofusers","NUMBEROFACTIVEUSERS":"numberofactiveusers","NUMBEROFARTICLES":"numberofarticles","NUMBEROFFILES":"numberoffiles","NUMBEROFADMINS":"numberofadmins","NUMBERINGROUP":"numberingroup","NUMINGROUP":"numberingroup","NUMBEROFEDITS":"numberofedits","DEFAULTSORT":"defaultsort","DEFAULTSORTKEY":"defaultsort","DEFAULTCATEGORYSORT":"defaultsort","PAGESINCATEGORY":"pagesincategory","PAGESINCAT":"pagesincategory","PAGESIZE":"pagesize","PROTECTIONLEVEL":"protectionlevel","PROTECTIONEXPIRY":"protectionexpiry","NAMESPACEE":"namespacee","NAMESPACENUMBER":"namespacenumber","TALKSPACE":"talkspace","TALKSPACEE":"talkspacee","SUBJECTSPACE":"subjectspace","ARTICLESPACE":"subjectspace","SUBJECTSPACEE":"subjectspacee","ARTICLESPACEE":"subjectspacee","PAGENAME":"pagename","PAGENAMEE":"pagenamee","FULLPAGENAME":"fullpagename","FULLPAGENAMEE":"fullpagenamee","ROOTPAGENAME":"rootpagename","ROOTPAGENAMEE":"rootpagenamee","BASEPAGENAME":"basepagename","BASEPAGENAMEE":"basepagenamee","SUBPAGENAME":"subpagename","SUBPAGENAMEE":"subpagenamee","TALKPAGENAME":"talkpagename","TALKPAGENAMEE":"talkpagenamee","SUBJECTPAGENAME":"subjectpagename","ARTICLEPAGENAME":"subjectpagename","SUBJECTPAGENAMEE":"subjectpagenamee","ARTICLEPAGENAMEE":"subjectpagenamee","REVISIONID":"revisionid","REVISIONDAY":"revisionday","REVISIONDAY2":"revisionday2","REVISIONMONTH":"revisionmonth","REVISIONMONTH1":"revisionmonth1","REVISIONYEAR":"revisionyear","REVISIONTIMESTAMP":"revisiontimestamp","REVISIONUSER":"revisionuser","CASCADINGSOURCES":"cascadingsources","NAMESPACE":"namespace","DISPLAYTITLE":"displaytitle","!":"!","CURRENTMONTH":"currentmonth","CURRENTMONTH2":"currentmonth","CURRENTMONTH1":"currentmonth1","CURRENTMONTHNAME":"currentmonthname","CURRENTMONTHNAMEGEN":"currentmonthnamegen","CURRENTMONTHABBREV":"currentmonthabbrev","CURRENTDAY":"currentday","CURRENTDAY2":"currentday2","CURRENTDAYNAME":"currentdayname","CURRENTYEAR":"currentyear","CURRENTTIME":"currenttime","CURRENTHOUR":"currenthour","LOCALMONTH":"localmonth","LOCALMONTH2":"localmonth","LOCALMONTH1":"localmonth1","LOCALMONTHNAME":"localmonthname","LOCALMONTHNAMEGEN":"localmonthnamegen","LOCALMONTHABBREV":"localmonthabbrev","LOCALDAY":"localday","LOCALDAY2":"localday2","LOCALDAYNAME":"localdayname","LOCALYEAR":"localyear","LOCALTIME":"localtime","LOCALHOUR":"localhour","SITENAME":"sitename","CURRENTWEEK":"currentweek","CURRENTDOW":"currentdow","LOCALWEEK":"localweek","LOCALDOW":"localdow","REVISIONSIZE":"revis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At least url protocols is redundant given wgUrlProtocols is already available on all pages via the startup module.
In addition, beware of the caveat of using page output: HTTP caching. Using a data module is better for cache consistency, given that data modules will always atomically share the same snapshot in time as the javascript code, whereas use of page output means the config values are stuck in static http cache, but the JS code will update independently, which can certainly cause maintenance complications (or bugs).