I'm developing an objective-j app right now and they have compile targets that compile directly to minified javascript. It will work without compiling but for a production I wouldn't recommend it for bandwidth reasons alone. Technically they are right the javascript is still interpreted in the client and you don't need any browser plugins to run it. It's not strictly objective-j source code your are running in a production build so there aren't too interpreter passes.
The Debug build targets don't compile to javascript though so there are two interpreter passes in that case. That is fairly speedy and I don't notice an insane startup difference.
Actually, the main difference between debug and release are that release builds also minify/compress the code. Both compile from objj to js.
Of course, you can not run the compiler at all and the code will still run in your browser, because it will do the compilation "just in time". There's not a significant performance penalty for doing this, and browser refreshes are still speedy. But when you're ready to deploy, you might as well build ahead of time even for a 10-20% launch time speedup.
The Debug build targets don't compile to javascript though so there are two interpreter passes in that case. That is fairly speedy and I don't notice an insane startup difference.