A lot of users complained about the long time the diffs need to load with the Pending Changes. So I tried to find out more infos using Firefox's Firebug add-on. Here is what I get:
As of now, when reading articles like "Alexander the Great" I get the normal behaviour (2-3 seconds to load the page).
But when I try view a diff, it's another story. There is only one issue: the first request for the diff (http://en.wikipedia.org/w/index.php?title=Alexander_the_Great&diff=369978513&oldid=369978381). It was only 82 Kb to download, and this request had to wait 18 seconds for the servers to begin sending the page. Once it started downloading, the download of the rest of the page began normally. All requests and elements on the page loads normally, with a total of 2 or 3 seconds.
There is a huge difference between the loading length of the article (3 seconds max) and it's diff (up to 21 seconds).
I'm not sure it's a pending changes issue though. I have the same issue on "London" for example. The first diff request had to wait 18 seconds, and was aborted. A new diff request was made, and I had to wait 30 seconds for the download to begin.
Anyway, one thing is certain: there is an issue with the diffs. You might want to download Firebug and try to reproduce.
Version: 1.17.x
Severity: normal