Our 1.9 logic seems to be searching through filesystem to try and determine the canonical path we are storing our loads under in features. It appears MRI is not doing this. This little program (make an 'a' and 'b' directory both containing a 'test.rb' with different contents) demonstrates us double loading where MRI will not (tried against MRI 1.9.2 and 1.9.3):
$LOAD_PATH << 'a'
$LOAD_PATH = 'b'
Our resolution process may be a significant load cost for things like rails which has a lot of the same requires sprinkled around their codebase.