JIT (Just-In-Time) compiler was first introduced as an experimental feature in Ruby 2.6 with the intention of improving the performance of Ruby programs. Being experimental, memory-intensive workloads such as Rails application could not benefit from it at the moment of it’s introduction, despite achieving 1.7x faster performance as compared to Ruby 2.5.
In a similar fashion, Ruby 3.1 has introduced YJIT by Shopify. The technology is aimed at improving the performance of Ruby applications. However, being an experimental feature, it is disabled by default and has to be explicitly specified in order to be used. The command-line option to enable it is
--yjit. The following resources would be helpful: