Android 2.2 and later uses Google's V8 JIT. iOS 4.3 and later uses Apple's Nitro JIT. Firefox 4 and Opera 11 also have JIT compilers enabled for ARM devices.
Mobile processors are still slower than desktop ones, of course, but JavaScript on mobile today is already faster than JavaScript on desktop just a few years ago. For example, the benchmarks on http://arewefastyet.com/ show V8 running only about 5x slower on an nVidia Tegra 250 ARM board than it does on an Intel Core 2 Duo Mac Mini.
Mobile processors are still slower than desktop ones, of course, but JavaScript on mobile today is already faster than JavaScript on desktop just a few years ago. For example, the benchmarks on http://arewefastyet.com/ show V8 running only about 5x slower on an nVidia Tegra 250 ARM board than it does on an Intel Core 2 Duo Mac Mini.