How might the acquisition of language account for the very fast cultural and intellectual development that has occurred in the evolution of humans?

There's actually no good answer to this question, because we don't know whether it was very fast or not. We are the only species to have evolved speech, so we really can't compare to see whether we were fast or slow.

As it is, we'd expect cultural evolution to be many times faster than biological evolution, because it doesn't follow the same rules. Biological evolution is limited to vertical gene transfer; culture can spread its memes - ideas, habits, language patterns, etc, far and wide without having to wait for a new generation to emerge.