Which JavaScript graphics library has the best performance?

Raphael JavaScript Library

http://raphaeljs.com

None of them have good performance. It is 2009 and the state of programmable graphics rendering in web browsers is truly depressing. I could do faster interactivity on a vt125 terminal 25 years ago. If you are doing anything interactive, think about using Flash... Else I'd go for some server-side heavy solution depending on your needs

Updated answer (2019):

The core advice is still the same: for maximal performance use thin wrappers or use raw browser API's, and also avoid the DOM or any DOM-like structure. In 2019 this means avoiding SVG (and any library built on top of it) because it may cause performance issues when trying to rapidly change the DOM.

Canvas is the go-to solution for high performance web graphics, both for the 2d and 3d (webgl) contexts. Flash is dead so no longer an option, but even if it weren't its performance was eventually matched by the native browser API's.

Original answer (2009):

If you're not doing 3d, just use raw canvas with excanvas as an explorer fall-back. Your bottleneck will be javascript execution speed, not line rendering speed. Except for IE, which will bog down when the scene gets too complex because VML actually builds a dom.

If you're really worried about performance though, definitely go with flash and write the whole thing in actionscript. You'll get an order of magnitude better performance, and with the flex sdk you don't even need to buy anything. There are several decent libraries for 3d in flash/flex available.