Faststone Image Viewer displays the embedded JPEG preview inside the raw NEF file. The in-camera processed preview has default NR and Sharpening applied, which appears to be more aggressive than what you are applying inside LR. When viewing high ISO images at less than 1:1 view with insufficient Luminance Noise Reduction applied the Develop module preview is not accurate. When the image is exported with resizing it will appear lighter and perhaps less sharp than what was observed at the same image size (i.e. less than 1:1) in the Develop module.
Try adjusting the LR NR controls at 1:1 view to reduce visible Color and Luminance noise. Readjust Sharpening as required, but try not to use overly high settings (i.e. 50 Amount and 35 Detail or less). The Library module preview is more accurate (than the Develop module) with high ISO images when set to a view size close to the the resized export image resolution.