Rails I18n lookups are uncached by default, which can lead to huge slowdowns especially when rendering tables with eg. localized dates.
There is a big speedup if the memoized backend is used, which caches lookups in memory. I usually have the following code in an initializer:
I18n::Backend::Simple.send(:include, I18n::Backend::Memoize) unless Rails.env.development?
This tip might be worth adding to the rails performance section.