1.8.7 の require 'addressable/uri' 自体は一瞬なので、100倍というか…
$ time ruby1.9 -rubygems -e "require 'addressable/uri'"
14.49s user 0.13s system 99% cpu 14.678 total
$ time ruby1.9 -rubygems -e "require 'addressable/uri'"
14.49s user 0.17s system 100% cpu 14.648 total

$ time ruby -rubygems -e "require 'addressable/uri'"
1.58s user 0.33s system 25% cpu 7.560 total
$ time ruby -rubygems -e "require 'addressable/uri'"
1.44s user 0.28s system 100% cpu 1.719 total