In the source code it explicitly turns off optimization with this line:

cx.setOptimizationLevel(-1);

This is because env.js is too large for the 64KB bytecode limit. However, if you split the files up like what is suggested here you can turn back on optimization and get better performance:

http://stackoverflow.com/questions/9779809/how-to-split-iife-to-fit-into-java-jvm-rhino-64k-bytecode-limit