I have setup my puma web server based on heroku manual: Deploying Rails Applications with the Puma Web Server
workers Integer(ENV['WEB_CONCURRENCY'] || 2)
threads_count = Integer(ENV['RAILS_MAX_THREADS'] || 2)
threads threads_count, threads_count
port ENV['PORT'] || 3000
environment ENV['RACK_ENV'] || 'development'
web: bundle exec puma -C config/puma.rb
rails s does not use the Procfile, you may want to look at using a tool like foreman: https://github.com/ddollar/foreman
update: this was the solution:
config.allow_concurrency = true in the rails development config.