quiet_assets_middleware.rb

# config/initializers/quiet_assets.rb
class QuietAssets
  def initialize(app)
    @app = app
  end

  def call(env)
    original_level = Rails.logger.level
    if env['PATH_INFO'] =~ %r{\A/assets/}
      Rails.logger.level = Logger::WARN
    end
    @app.call(env)
  ensure
    Rails.logger.level = original_level
  end
end

# config/development.rb
config.middleware.insert_before 'Rails::Rack::Logger', 'QuietAssets'