Если вы используете rails 4, вы можете применить следующий патч, и webrick начнет обслуживать запросы одновременно.
diff --git a/railties/lib/rails/commands/server.rb b/railties/lib/rails/commands/server.rb
index e3119ec..ef04aa8 100644
--- a/railties/lib/rails/commands/server.rb
+++ b/railties/lib/rails/commands/server.rb
@@ -93,14 +93,6 @@ module Rails
middlewares << [Rails::Rack::Debugger] if options[:debugger]
middlewares << [::Rack::ContentLength]
- # FIXME: add Rack::Lock in the case people are using webrick.
- # This is to remain backwards compatible for those who are
- # running webrick in production. We should consider removing this
- # in development.
- if server.name == 'Rack::Handler::WEBrick'
- middlewares << [::Rack::Lock]
- end
-
Hash.new(middlewares)
end