Although partially answered on the index page, here is a more complete answer: If you manage massive amounts of comments, Isso is a really bad choice. Isso is designed to be simple and easy to setup, it is not optimized for high-traffic websites (use a dedicated Disqus instance then).
Comments are not big data.
For example, if you have 209 threads and 778 comments in total this only needs 620 kilobytes of memory. This is an excellent use case for SQLite.