Translate extensions uses super simple in-process caching on translate_metadata. Every time a caller wants to read something from that table, the code loads and caches the whole table into process memory. The good side of this is, when the table is small, that it will only cause one extra query instead of a lot of them. This way they callers do not need to care or do preloading.
However, on Meta the table has grown to 22k entries, and MediaWiki has started to warn about big reads from the database. Fixing this properly requires checking all callers and add suitable preloading to those who need it.
Aside, for testing and other purposes, this class shouldn't be static, but a service.
Outcome
Database query performance issues related to translate_metadata table have been resolved.