Render Cache Color Output

Hi,
I'm unable to "Render Cache Color Output" in Resolve 15.
Timeline plays back at 5fps and there is no red bar above the timeline telling me I need to render.
Note: if I turn off my "noise reduction" node, play back is good at 23.98.
Thanks

Have you set the render cache mode to 'user' in the playback menu?

Yes I have.