How to optimize a Full Text Search
From Performance analysis of MySQL's FULLTEXT indexes and LIKE queries for full text search by Henning Koch:


  FULLTEXT performs better when your text has low redundancy
  FULLTEXT performance differs by a factor of 78 between a vocabulary of 1,000 words and 100,000 words. I guess that larger vocabularies result in a very wide but shallow inverted index that can quickly determine if a query has matches or not. An educated person has a passive vocabulary of 15,000 to 20,000 words, so FULLTEXT should work well for natural language texts.