Although partially answered on the index page, here 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, not optimizied for high-traffic websites (use a dedicated Disqus instance then).
comments are not big data
For example, 209 threads and 778 comments in total only need 620K (kilobyte) memory. Excellent use case for SQLite.