Use Nodes issue

I’m trying to composite a still image, but every time I click on “Use Nodes”, the image node starts acting weird/blinking. Then at the top of the toolbar it says it’s trying to cancel something with the percentages fluctuating from 0 to 100 here. My render result ends up going straight white or gray.
Prior to this I was rendering an animation straight to mp4 format just so I can quickly preview it. Instead of compositing an entire image sequence I want to composite one still of it, but the nodes start having issues. This isn’t the first time this has occurred.