yeah, but "return immediately if string is in $LOADED_FEATURES"" isn't actually always a fast operation in ruby.
the speed of require can be proportionate to the size of $LOADED_FEATURES and in applications with a big stack and dependency graph (like Rails) it can actually slow stuff down before the FS is touched.

it has been discussed here http://rhnh.net/2011/05/28/speeding-up-rails-startup-time