"Purely functional" is a misnomer. A better description is "typed effects". Haskell doesn't forego effects, it types them. You still have imperative destructive updates if you need them (e.g: to avoid a logarithmic complexity hit, or a constant performance hit). You still may want to write imperative programs, often-times as the backend of a pure functional interface.

Haskell is actually quite good at it, and jokingly referred to as "the world's finest imperative language".

Haskell doesn't have auto-parallelism. There are some excellent libraries for parallelism (strategies, DPH, repa) but they take some work. True auto-parallelism (a la Parasail) is far more difficult to get right (often due to granularity of work units).

Mercury does a decent job of this (not quite auto-parallelism, you need to hint at parallelization points, but it manages granularity for you).