Modern JS engines are fast enough for many tasks not possible before. For example, I implemented AES256 and SHA256 in javascript for "security locker" website

Modern JS engines are fast enough for many tasks not possible before. For example, I implemented AES256 and SHA256 in javascript for "security locker" website (http://www.memengo.com http://www.memengo.com). I spent few days hand-optimizing the implementation to make it fast enough to encrypt user data in slow browsers like IE6, but last-generation browsers like FF3.5, Safari 4, Chrome would be fast enough even without excessive tuning.