How does ARM Cortex A8 compare with a modern x86 processor

There are some benchmarks that compute scores that can be used for such comparisons. Try searching for DMIPS/MHz and maybe also CoreMark/MHz.

I'd expect them to be more comparable in performance to Atom CPUs though.

What you're looking for is quite hard to find, mostly because you're comparing apples to oranges. You can't really compare load times for a webpage, or any other thing for that matter, mostly because these operations are not only CPU dependent, they're also dependent on the OS they're running and on the hardware that it uses (RAM being a major candidate).

Also, ARM chips are highly optimized for just a few specific tasks, while on a Core i7, for instance, you also have CPU operations used for complex stuff (AES encryption, video encoding, parallel processing) that an ARM CPU simply doesn't need to do.