what happens is when I add the denoiser to the render layer in compositor ,it hangs my laptop for roughly the same amount of time it would take for Blender to do it if was during rendering so it is not clearly working as I asked people they told me that this is supposed to be faster solution.
That's not exactly how it works. It doesn't actually make the rendering go faster. The node uses an algorithm to reduce the amount of noise in the final image, in post processing, so it just allows you to use a much smaller number of 'render samples' that would usually result in a very noisy final image, and make the image 'smoother' (with less noise). Less samples = faster render.
(go into render settings tab, under "sampling" and reduce the number or "Render" samples)
- If the number of samples is too low the resulting image will tend to look blurry and smudged, so it's not good to reduce too much the number of samples.
- It's not so good with refractions, so if you're rendering objects made of glass or transparent liquids, it may look a little weird.
- the denoising node effect is only applied after the imagem is completely rendered, so it may still look noisy while it's rendering.
Edit: In the render properties tab there is also a denoise option that you can enable. It denoises the image while rendering. If that is enabled, you don't need to add the node in the composite, it won't work because the rendered image will already be denoised.