put in config/initializers to silence asset logging in Rails development mode

silence_assets.rb

if Rails.env.development?

  Rails.application.assets.logger = Logger.new('/dev/null')

  Rails::Rack::Logger.class_eval do
    def call_with_quiet_assets(env)
      previous_level = Rails.logger.level
      Rails.logger.level = Logger::ERROR if env['PATH_INFO'] =~ %r{^/assets/}
      call_without_quiet_assets(env)
    ensure
      Rails.logger.level = previous_level
    end
    alias_method_chain :call, :quiet_assets
  end

end

You can also silence asset logs with the "quiet_assets" gem.

gem 'quiet_assets', group: :development