Your manual would tell you how to turn it off or anything like that. According to an online manual for your camera, from
https://www.manualowl.com/m/Sony/DSC-H400/Manual/414649, it states that...
"When the shutter speed becomes slower than a certain speed, the NR slow shutter function is activated automatically to reduce image noise, and [NR] appears on the screen"
"If you select slow shutter speeds, it takes time to process the data."
So you would have to check your manual to see if you can turn off the NR, I would assume such features were only built-in to help hand holding at slower speeds, so they're not really must use things for when using tripods, therefore could be overridden or turned off. You need to find a way to turn it off, the only way you can tell is if the symbol NR is no longer showing on your LCD screen.