RingoJS is not slow. We use it in the Erbix backend, running a separate Rhino/RingoJS engine for each account. It's really stable, fast, multi-threaded.
We use it in conjunction with PostgreSQL JDBC driver, serving large source code files, rendering templates written in JavaScript, static files upload/download, etc.
http://www.erbix.com is an Erbix app running on a standard Erbix account.