There are several reports by editors using HHVM at https://www.mediawiki.org/wiki/Talk:HHVM/About and https://www.mediawiki.org/wiki/Talk:HHVM ; 503 error messages appear while trying to load huge pages.
See https://www.mediawiki.org/wiki/Talk:HHVM#Big_Pages_and_Lua_Timeout for a list , I can confirm the same thing happens to me for several of those pages on Firefox.
Version: wmf-deployment
Severity: major