Crypto libraries and speed in client side javascript
This page has some speed tests:
I am not up to speed (so to speak) on what performance repercussions the different settings for AES has, as presented on the page, but the forge library stands out as being five times faster at encryption and 2,5 times faster at decryption than any other library (as tested on a Linux Firefox 22.0 on a one core Celeron) and ten times faster than CryptoJS.
A library not included in the test that may be worth checking out is cifre.