Why Mobile Web Apps Are Slow

Great articled on Sealed Abstract on this. Go read the whole thing - its long (10K words!), but completely absolutely worth it.
The conclusion is fairly straightforward (and depressing)

So here’s what you should remember:

• Javascript is too slow for mobile app use in 2013 (e.g., for photo editing etc.).  

• It’s slower than native code by about 5

• It’s comparable to IE8

• It’s slower than x86 C/C++ by about 50

• It’s slower than server-side Java/Ruby/Python/C# by a factor of about 10 if your program fits in 35MB, and it degrades exponentially from there

• The most viable path for it to get faster is by pushing the hardware to desktop-level performance.  This might be viable long-term, but it’s looking like a pretty long wait.

• The language itself doesn’t seem to be getting faster these days, and people who are working on it are saying that with the current language and APIs, it will never be as fast as native code

• Garbage collection is exponentially bad in a memory-constrained environment.  It is way, way worse than it is in desktop-class or server-class environments.

• Every competent mobile developer, whether they use a GCed environment or not, spends a great deal of time thinking about the memory performance of the target device

• JavaScript, as it currently exists, is fundamentally opposed to even allowing developers to think about the memory performance of the target device

• If they did change their minds and allowed developers to think about memory, experience suggests this is a technically hard problem.

• asm.js show some promise, but even if they win you will be using C/C++ or similar “backwards” language as a frontend, rather than something dynamic like JavaScript