Since I switched to a static RSS feed at MacHack (thanks Jon :), I’ve noticed a dramatic decrease in bandwidth usage.
Backend.php was accounting for 37% of my hits and almost 56% of the bandwidth usage. Each time a user updated the RSS feed, it had to execute several SQL queries to generate the file, and it would always return the entire feed. With a static file, if it didn’t change since the last time a user requested it, they’ll get a 304 and no data will be sent, which further reduces the bandwidth.