There are some optimization ideas in this e-mail thread (especially in the leaves or the thread).
Basically, Buffers have no accessable constructors, so the compiler knows that there can be no user classes derived from Buffer. Buffers can be treated much like final classes and all Buffer methods could be inlined.
Many of the get and put methods could be inlined as if they were fancy array accesses.
Closing as won't fix as the Java front-end has been removed from the trunk.