If you installed 1.9.3-p125 update your rubygems.

gistfile1.txt

ruby -v                                                                                     
ruby 1.9.3p125 (2012-02-16 revision 34643) [x86_64-darwin11.3.0]

gem -v
1.8.11

time ruby -r active_record -e ActiveRecord::Base                                                                                              
2.05s user 0.05s system 99% cpu 2.101 total

gem -v
1.8.23

time ruby -r active_record -e ActiveRecord::Base
0.29s user 0.04s system 98% cpu 0.336 total