Identified this in profiling a multilingual site. isEnabled() loads the config entity (or even creates one on-demand if it doesn't explicitly exist). This has a non-trivial overhead because config entities by default aren't statically cached.
isEnabled() is called a lot, content_translation_language_fallback_candidates_entity_view_alter() calls it, and that can be called dozens to hundreds of times per page, because
There are multiple options.
It might already help a lot to just enable static caching on that config entity type, might require some test fixes, we'll see.
We could also use a full cache collector approach that persistently caches this information.
Needs work
11.0 🔥
It affects performance. It is often combined with the Needs profiling tag.