You bring up a good point. I've noticed that their servers seem to be under a high load when it's morning on the West Coast (of the USA). However, during the evenings, downloads seem to go much faster. This knowledge + a well-placed sleep(2) statement should prevent anyone from harming this server.
Comment on Re: Use WWW::Mechanize to Download Pictures of Sayuri Anzu
I recommend falling on the side of over-politeness b/c it's more likely to keep resources open and less likely to inspire lots of new tools to block this kind of thing. I usually sleep 30 seconds or more per domain (requests to different sites don't really need sleep). The big cats like Google go even longer with their bots.