"Gravatar Sucks …" could the icons be mirrored / archived / cached on the forum server? That, I would think, would speed them up. Maybe. Or not. Same for JPGs. Though not videos.
Just talking, brain disengaged.
Well, the problem is this. The forum software itself goes and looks for relevant details about each user who has posted, which is (at best, if the software's coded properly) one database hit. Part of the data returned is information about the avatar (usually the name of a file stored on the server's filesystem, might be information required to make a second database query and return the binary image from the database, or, in the current case, a key that's used to go off and get the image from gravatar.com). This information is used to produce an <img> tag, which is sent back to the browser. As the page is rendered, the <img> tag is resolved, and a query is made. For the "server-local storage" case, the browser knows where the server is (no DNS lookup, so faster), and should already have a connection to the server open (which also cuts time). With the gravatar.com case, a dns lookup and new connection have to be established. And then there's the latency of gravatar.com, which appears to be significant when there's a large number of queries being made.
I just profiled the FAQs page, it took 41 seconds to load.
The HTML for the page itself claims to have been built in 4.9 seconds, which looks about right (but which is glacially slow anyway).
The remaining 30+ seconds was *entirely* made up of requests to gravatar.com. 1089 requests to gravatar.com.
Even storing them locally wouldn't fix this issue – 1000+ requests take time to serve, even if it's by returning a "304 Not Modified" response, and especially given that each hit to the faqs page would then cause another 1000+ hits to the raspberrypi.org server, which appears to be falling apart at the seams already.
Avatar gettage is far less significant on pages with a "sensible" number of comments, of course – the FAQs page as it stands is totally pathological and needs to have its comments removed.