Very slow page generation, unable to find cause

This is only really happening in my post editor screen (Gutenberg). The page itself loads relatively quickly, but the categories and tags take a very long time to load in. Sometimes up to ten seconds. I don’t have many entries in either the categories or tags.

I’ve left the monitor window open when I load the editor, as the top bar disappears, but I can’t see anything obvious in there. Is there anything I can use in Query Monitor to help me find the cause of the delay?