Actually WPFTS could be not that great for you in case you have millions of records (e.g. posts).
The whole Wordpress could be not good for you either, because to handle such amount of data you will need to use specific optimization and caching algorithms and even clustering.
I would like to propose you to use full-text solutions based on Apache Solr or Elastic Search - those tools are implemented in Java and work way faster than PHP+MySQL solution provided by WPFTS and they are especially good for big data.
Alternatively you can wait until we release a new WPFTS version (in 1-2 months) that can use self-hosted Elastic Search engine to keep full-text index and uses WPFTS data collecting algorithms.
Thank you for the question.