I found 2 different references about IndexedDB in Mozilla website.
Statement 1: "IndexedDB is an API for client-side storage of significant amounts of structured data"
Statement 2: "IndexedDB is a transactional database system, like a SQL-based RDBMS;"
If I am correct then IndexedDB is a database API for underlying browser embedded database, like LevelDB for Chrome.
It is important to understand what IndexedDB, if want to look from performance perspective - IndexedDB Performance and IndexedDB v/s WebSQL performance comparison