Says who? Python, for one, is plenty usable, and is not fast.
>Which it isn't
Says who? JS engines are close to Java/C in speed, and 10x faster thn Ruby, Python et al.
V8 is faster or as fast than statically compiled Go or JITed Julia, for example...
In any environment where I might currently choose to use Python I also have the option to use something else for parts of the project where Python proves to be too slow. Will FirefoxOS provide such an escape hatch?
>JS engines are close to Java/C in speed
'close' is a pretty vague term. For a lot of tasks Ruby is 'close' enough to C that the difference doesn't matter. For a different set of tasks Java is not 'close' enough to C (or C+asm) to be a viable choice and neither is Javascript.
I'd also like to point out that battery life does matter, and using at least twice the CPU cycles for most tasks isn't conducive to good battery life.
Seeing that Python is 10-20 times slower than V8 for most Python/JS native operations, you should have that problem much. Especially considering that the purpose of asm.js is to give you an even greater boost in speed. And seeing that NaCl never got anywhere, not only this is your best bet but it's far better than anything else out there at the moment.
asm.js IS a bytecode format. That it is human readable or that it accepts some tradeoffs because of JS doesn't matter. The end result (after the JIT pass) would not be any slower for it. The only problem with a readable "asm" would be slower load times, but that can be taken care of in the future by providing some pre-compiled format or more control over caching if asm succeeds.
There are only a very small number of languages that can legitimately claim that (C++, Fortran, and sometimes Ada). Java is not one of them. JavaScript is surely not one of them, even with the latest versions of V8.
The only time we see performance remotely close (which still usually means several times slower, at best) to C is for extremely unrealistic micro-benchmarks that have been very heavily optimized to a state where they don't at all resemble real-world code.
We have had Word Processors and Spreadsheets in Javascript (Google Docs), 3D and 2D games, and even a h264 decoder and a PDF renderer. Heck, they have ported QT to Javascript, and the example applications run at a very acceptable speed. None of the above are slow.
So, no, it's not true that V8 is only fast in selected "microbenchmarks".
You might not do scientific applications or NLE video editing with it, but for everything else it should be just fine.
Where does the idea come that V8s and co very acceptable speeds come at the expense of battery life and wall-clock performance???
Not to mention that people are using far less capable web apps in the mobile and desktop space now (i.e pre-asm.js javascript), so the increase in speed due to the asm.js/optimisation standardisation would only make battery life and wall-clock performance better.