Strange, mine is set to ON, yet when i was out the other day, the images showed on the lcd display instantly, this was with th IR filter and 2-3 min exposures
Edit : Have just tried again, seems to make no differance to the length of exposure whether the Long exp NR is on or off Hmmnnnnnn
Edit Again : Just read the manual, and it says
"Noise reduction is not performed on continuous shooting or continuous bracketing even when it is set to on"
That will explain it then !