Thanks Beppe, that makes sense.

Thanks for your reply but as I mentioned the "denoise on completion" is on, I never turned it off. Even if it's turned off I wouldn't expect a slow down by 60 fold. I noticed that there's a correlation between the used Vram and this issue but I might be wrong.

I don't know if this is a bug but what's happening is that when I turn the denoiser on it slows down the render from about 5-6 ms/sec to 0.1 ms/sec and slows down the whole system (denoise on completion is on) this isn't happening on one specific scene, I tried it with several scenes.
I should say ...