Denoise takes FOREVER to process. Are you doing a proper preview or are you thinking to yourself, "I know how this AE works! I'll just hit the space bar!"
Wrong. Hit zero on the numeric keypad. And wait. This is AE. Not a video editor.
Tried both methods - space bar and zero and it just crawls.
I've installed the latest CUDO drivers.
This makes me nervous - Do you have a compatible NVIDIA GPU? If not uninstall the drivers right now.
Are you new to AE? A ram preview has to render. The speed of the render depends on the Composition Panel resolution. If it is set to Full and you are working with HD footage (1920 X 1080 @ 29.96 fps) it can easily take several seconds a frame to render the preview. Once the preview is rendered the comp should playback at full speed. If your comp is 60fps it will take at least twice as long to render at Full resolution. If it is 4K it will take at least 4 times longer to render.
The best option, especially if you are working on a HR display, is to set Composition resolution to Auto and set the Magnification ratio to 50% for previews. Denoise is hard to judge at less than Full resolution and 100% (200% for HR displays) so the fastest thing to do is to pick a real noisy part of the frame and preview using Region of Interest. If that looks OK then close the region of interest and the render a suitable DI (digital intermediate using a suitable production codec - NOT h.264 or any other delivery format) for use in the rest of your production. Because Denoise is so slow to render it does not make much sense to apply Denoise and then add a bunch of other effects and edits to the comp. Previews are so slow that it becomes nearly impossible to get anything done. Denoise>Render a DI>Import and replace usage in your project and go from there.