I'm not sure about performance because I didn't test with very large amounts of data, but despite my best effort to integrate TNTSearch with Scout using the official package, I was not able to get half of their supposed features to work during any of my tests.
So feature-wise it wasn't really bringing anything over the built-in search, but it could technically be a bit more performant just by avoiding the full text index search. But I don't know how it works under the hood so it could also be less performant than Flarum built-in search.
Meilisearch however does bring significant feature improvements and has the potential to run much faster than the built-in search. I'm not sure how much ressources would be necessary to handle that many posts with ease though. It'll beat the built-in search for sure but no idea how that compares with ElasticSearch.
