Denoise - cache won't playback

When using "denoise" on clips, I've cached the clip, it indicates blue on the timeline, as if it's ready for playback, but still won't play in real time. If I remove the "denoise" node, it plays fine.
Will the "denoise" tool not cache? how do you work around this issue?
Thanks
Tim

You could select the NR node and manually force that to cache by right-clicking and choosing User. I get mixed results with Smart Caching, which doesn't always catch the NR nodes. Forcing it with User always works.