In order to have a transition path to this coming big change, it was decided to have a magic comment at the beginning of files, so you can use in Ruby 2.3.
To do so, just add this comment in the first line of your files:
# frozen_string_literal: true class YourClass # ... end
Trying adding it to your spec_helper file and let me know in the comments if you performance improved!
More info: Ruby issue #8976.