The long processing time is the camera capturing a second exposure with the shutter closed. It will subtract the noise pattern from that exposure from the image you took. You can switch it off in the settings if you want, and there will be no processing delay.
I have long exposure NR turned off. It’d take an equal amount of time as the exposure if that were the case - a 2 min black frame for a 2 min exposure. Not 8 secs for exposures 1 min and above as I have been observing.
