In both Lightroom and ACR, the Denoise amount cannot be adjusted before applying it. This is a regression from the previous preview UI where the amount could be changed before applying Denoise.
This means that Denoise must be applied before changes are possible, and the result must be recalculated if a change IS made. This is very, very poor behavior and a huge waste of time for anyone using (for example) different cameras or ISO settings which may need different Denoise amounts. This is made worse because it appears that Denoise amount always starts at 50 and doesn't use the previous setting (unless that can be changed.)
It also appears that this only happens if multiple files are selected, and the most selected file is NOT updated? If I select three files with Auto-Sync, apply Denoise, change the slider, the update dialog only shows two files being updated.