FCPx freezing when rendering after every tiny edit

Hi all, hoping you can help me solve this one!

Every once in a while, FCPx gets to a stage where it will render and freeze the whole application after every small edit I make. I'm talking about typing one letter, or moving a graphic up a few pixels.

I feel like I was able to solve this ages ago, but I can't remember how I did it! Any ideas on what is causing this and what I can do to fix it? I've tried rebooting the app, updating it in the app store, rebooting and updating the iMac itself, deleting the generated library files... but no luck.

TIA!!