Use frozen_string_literal:

frozen_string_literal reduces the generated garbage by ~100MB or ~20%! Free performance by adding a one line comment.

Conclusion
Gem authors: add # frozen_string_literal: true to the top of all Ruby files in a gem. It gives a free performance improvement to all your users as long as you don’t use String mutation.

Mike Perham: Ruby Optimization with One Magic Comment, 2018-02-28: https://www.mikeperham.com/2018/02/28/ruby-optimization-with-one-magic-comment/

Use Symbol as hash keys:

If you use Ruby 2.2, Symbol could be more performant than String as Hash keys.

Fast Ruby: https://github.com/fastruby/fast-ruby#hash