Someone suggested an edit to one of my old answers on Stack Overflow. When I followed the notification, I noticed that loading the suggested edit took quite a bit longer than I expected. I reloaded this a bunch of times, and the POST request to
https://stackoverflow.com/review/next-task/[id] did consistently take around 3 seconds to respond.
I also looked at the review queues then, as this looked like they share an endpoint. Those also took around three seconds to load. But the interesting part is that once I accepted the suggested edit, the review queues became fast again. So the presence of that suggested edit might have caused this slowdown for me. So of course I can no longer reproduce this now, but this was a very consistent slowdown while I could observe it.