Only a small subset of the Java library is emulated for use on the
client side; see
http://code.google.com/docreader/#p=google-web-toolkit-doc-1-5&t=RefJreEmulation.
You can use any library you desire for server-side code, but on the
client everything has to be properly emulated to handle javascript and
browser compatibility issues, so is limited to the above set. The "4x
better performance" might be true in Java, but does not correlate at
all to how it might be implemented in js!