Thats a good question. =)First question... Do you know what "Noise" is?

My camera has a 'long exposure noise reduction' feature...but it causes the camera to freeze up after the exposure, for about as long as the exposure. So for a 10 second exposure, it would freeze up for 10 seconds before letting me shoot again.
I turned that feature off 6 years ago.
Page 134 of your D3100 users manual explains that setting.