It takes a little bit of time for CNX to apply all of the adjustments in your raw file to the image it actually displays.That delay that you see is that little bit of time.
The raw file is just data that must be interpreted by CNX to a displayed image. Aside from the basic image date, the raw (NEF) file contains the instructions for the raw converter to interpret and then display the adjusted image. Depending on your file size, camera settings, adjustments that you have made in CNX and the speed of your computer, that processing time can be almost instantaneous, or take a second or two.
If it is the first time you open a NEF file in CNX, it creates a preview JPG that will be embedded in the NEF once you save it, unless you've changed this option in CNX. You'll notice a significant increase in your NEF file if you open it, then save it with no additional adjustments. This preview also takes some time to generate, and I suspect is better processed than the thumbnail JPG generated by the camera itself.
Best bet, after you open an image, or make an adjustment, let CNX "settle" for a 2 or 3 seconds before you judge the results. Then you can easily adjust contrast and sharpness in CNX.