Why mobile web apps are slow

Drew Crawford, in a long but well-researched essay on mobile app performance:

Think about iPhone 4S web development as [an]...environment that runs at 1/50th the speed of its desktop counterpart.  Per the benchmarks, you incur a 10x performance penalty for being ARM, and another 5x performance penalty for being JavaScript. Now weigh the pros and cons of working in a non-JavaScript environment that is merely 10x slower than the desktop.