Van Smith wrote an awesome articlecomparing current ARM processors and their lower power consuming x86 friends such as the Intel Atom.
Here's the conclusion of his performance benchmark tests:
"The ARM Cortex-A8 achieves surprisingly competitive performance >across many integer-based benchmarks while consuming power at levels far >below the most energy miserly x86 CPU, the Intel Atom. In fact, the ARM >Cortex-A8 matched or even beat the Intel Atom N450 across a significant >number of our integer-based tests, especially when compensating for the >Atom’s 25 percent clock speed advantage." > > However, the ARM Cortex-A8 sample that we tested in the form of the >Freescale i.MX515 lived in an ecosystem that was not competitive with >the x86 rivals in this comparison. The video subsystem is very limited. >Memory support is a very slow 32-bit, DDR2-200MHz. >
Languishing across all of the JavaScript benchmarks, the ARM Cortex-A8 >was only one-third to one-half as fast as the x86 competition. However, >this might partially be a result of the very slow memory subsystem that >burdened the ARM core. > >More troubling is the unacceptably poor double-precision floating-point >throughput of the ARM Cortex-A8. While floating-point performance isn’t >important to all tasks and is certainly not as important as integer >performance, it cannot be ignored if ARM wants its products to successfully migrate upwards into traditional x86-dominated market >spaces."