The geoip plugin is using v1.3.5 of the GeoIP gem (http://github.com/cjheath/geoip). v1.4 of the GeoIP plugin supports caching the entire database. I'm not sure how much of a performance improvement this will be, but it's got to be better than reading the entire file every time.
In my initial tests with my config, I see about 1250 messages per second without the GeoIP plugin, and about 1000 messsages per second as soon as the GeoIP plugin is enabled.
Closed in favor of the PR