I tried the new Denoise function immediately after it was released on some images I felt might benefit from its use. I was pleased with the results I received. After launching the Denoise function the initial screen where you could see the before and after results would launch in less than 10 seconds on all of the images I initially tried. I tried an image yesterday where the initial screen wouldn't show up after 10 minutes (at which point I cancelled it). This image had a lot of grass and small objects in it with an elk centrally located. I picked some other images that were close to it and all of them launched the review screen within 10 seconds. I looked at the edits against the Nikon raw file and decided to take a bunch of changes out starting where I added a mask and then had to subtract a number of elements from it. I added the mask back in with fewer subtractions then reran the Denoise function again. It then finished within 10 seconds.
I posted this in case anyone else runs across this issue, as modifications to edits made against the image might be necessary to get the Denoise function to complete properly in some circumstances. I'm guessing this is a bug that will be fixed at some point. Unfortunately, I thought I had saved the original edits so I could provide them as an example, but I lost them when I made the changes.