You know while raw speed is an important piece of a library, it is not the only thing, there are other factors that carry just as much weight when it comes to importance. 3rd party library ecosystem, community support, integration with other technologies, ease of use and a host of other are all just as important factors when I evaluate a library.

As well, IIRC Closure was an internal project that was built to build apps like Gmail, if that is the case then it, is reasonable to think that it has some cruft in their given that the state of the art in Javascript libraries came after Gmail, Oulook on the web, and other Browser based apps showed what was possible.

It was programmer transitioning from other languages to JavaScript that built these first toolkits and they brought over a good deal of their language constructs that they where familiar with as time went on other programmer from other disciplines joined in and some of the frameworks started to morph.

I remember when Dojo threw away their entire toolkit because of this and I commend them for doing so. They came to realize that their was a better way than just reimplementing Java or C# in the browser.

Closure on the other hand remained an internal project outside those learning. That being said, I do think their are much better frameworks available than Closure, Dojo and jQuery being two prime examples, but I do cut them some slack based on the fact that they would possible qualify as one of the oldest frameworks and that they did not benefit from the learning the communities went through as the state of the art evolved.