You can use profiling to list your 10 slowest examples:
spec -p spec/*/*_spec.rb --colour --format profile
If you run this on a specific test suite you can get the 10 slowest examples from a smaller subset of examples:
spec -p spec/models/user_spec.rb --format profile
Newer Versions of Rspec
For newer versions of rspec you should use the --profile option:
rspec spec/ --profile | (shows the 10 slowest examples)
rspec spec/ --profile 2 | (shows the 2 slowest examples)