Nerve cells don't need to work faster. Constructing conceptual thought in dreams is like building memories, or close; meaning that no actual actions happen; and considering that conceptual thought takes far less time for far more thought than regular thought, well, it really doesn't apply to the nervous system as such. Who knows...if I cared to make guesses I may even have theorized that conceptual thought is only part of one's "mind", and not one's "brain" (considering that I'm sure that the physical body and conscioness are different aspects of one being).

Some make it a point to train conceptual thought in their waking state and such a skill is surely benificial but far from simple. It seems to me that in dreams however, the ability for conceptual thought may be more...free, allowing its use in an easier way than during the waking state. Yet another chance for a simple experiment in a lucid dream (like testing to see if one has both waking and dreaming memory banks) - testing the use of conceptual thought and its ease during a lucid dream.

And the idea of why such long dreams don't have an enormous effect on one's mind/ego is also worth some studying I would suppose...