when should you seriously consider a dedicated search engine?

we've been using database full-text search for a while now, and honestly, it's just getting kind of slow with all the data we have. we're finding it hard to really tune the relevance and make queries fast enough. i'm curious, when did you all decide that you absolutely needed something like Elasticsearch or Solr, instead of just wishing you had it? what specific problems made you finally make the switch