What resolution are you rendering at, how many render elements that use denoising (such as light selects), and how much memory does your GPU have? I think it is likely to be something that can't be solved unfortunately, my guess is just that the GPU is taxed to its limit, probably due to using most of its memory, and when handling a larger resolution plus running all the windows in Windows and the viewports in Max, it just takes it a while to get everything cleaned up once done. We might be able to pass the information along to NVIDIA to see if there are any improvements that could be done; alternatively, a GPU with more memory might help. Of course, switching to the Intel or the High Quality denoisier would definitely avoid the lag.
EDIT you should be able to check your GPU memory usage in Task Manager, to see if it is indeed maxed out or close to it.