I've noticed that running Spring often causes 100% CPU usage, which results in fans spinning out of control. I've shared the script that I use for killing these processes, but that is a band-aid solution.
Instead of just killing the processes, why not prevent it from happening in the first place?
To do that, remove the listen gem from the Gemfile:
gem "listen"
gem "spring-watcher-listen"
Since Rails' default file watcher depends on this gem,
comment out this line in config/environments/development.rb:
# config.file_watcher = ActiveSupport::EventedFileUpdateChecker
This makes the fans much more silent.