In the trailblaze of creating the new term store, there's a really complicated logic built to get terms of entities (PropertyTermStore::getTerms() and copied logic in items, more context) but a) it's flawed, like "read new" for the term store is actually "read both" (let's see if the first one has it, if it doesn't, let's read it from the second one which is wasteful and would hide bugs like holes in the term store) and b) it's not used anywhere, if something needs all terms of an entity, it just loads it, that's actually way faster than hitting the term store and if you need the terms for just one language/term type. You would use prefetching term lookup instead.
We should get rid of this.