That's bizarre! Have you tried disabling everything except uBlock and seeing if that persists? Maybe it's some kind of interaction with other addons causing the problem.
Another user recommended uMatrix as a Ghostery replacement in this thread which I was trying out, and it looks like some Ghostery/uBlock interaction was to blame. My best guess is some Ghostery resource is correctly flagged by uBlock and that the delay I experienced was a request timeout or similar, from Ghostery.
That makes sense, and oddly enough I was just (literally for the first time, 10 minutes ago) looking at switching out Ghostery for uMatrix. Decision made, thanks.