A recent discovery is that you can actually continue to shoot while using
the in camera noise reduction- until the buffer is full. While you don't get
the satisfaction of immediate gratification, you don't need to wait for the
NR to finish before you take the next shot.
I let the camera finish the NR after about 3-4 shots, and then preview
them all at once. The accumulated NR processing does not seem to be as
long as the combined total of all the shots.
In camera NR does seem to be the best method for exposures over about
5-6 minutes, at least with Canon cameras.