In the last few days I've noticed that the "Flagged Posts" page has gone from loading instantly to hanging for 10-15 seconds before loading.
This is the only page on the SO site on which I am seeing this behavior. In fact, it's the only page on SO on which I have ever noticed a slowdown. As noted in the comments, I am now also seeing a slowdown on /flag-summary. Perhaps I didn't notice it before.
The server execution of the page itself appears to be the bottleneck (screenshot):
I see this behavior in Chrome, Firefox (PC, latest) and Safari (Mac, latest). I brought up the full site on my mobile phone (iPhone 5, outside of my home wireless network) and observed similar behavior.
This is making it a little inconvenient to efficiently review posts, as I often jump back and forth between questions and the tools page. Please let me know if I can provide more detail or assist in diagnosing the problem.
24 Hour Update: I've accessed the pages in question at least 50 times since posting this to a) perform review tasks and b) confirm that the behavior is still occurring. I've also added the iPad to the list of devices on which I see similar performance.
I also tested from a virtual machine hosted in one of the larger data centers in the US. Again, most of the site is fast but /tools ("flags" tab) exhibits the same load time of roughly 10 seconds.
Note that I have tagged this as "bug" only in the interest of taxonomy; I'm hesitant to classify poor performance as a true bug, especially as it may already be known/explained.