> I know Ruby wasn't created to make a fast-running language.
>
> But I just realized I've never seen any Ruby benchmarks, comparing it to
> other languages under heavy load or speed.
>
> Anyone seen some?

No.

> I ask for a real reason becuase it's under consideration to
> replace PHP with Ruby on a high-traffic website I work with.

As a complete non-expert, I suggest you try out FastCGI (just ask Ara
Howard) and squid (just to cache responses to take the heat off your app).
 If those technologies are amenable to your application, it wouldn't
matter if you wrote it in Commodore 64 BASIC.

Cheers,
Gavin