How to speed up Rails 5 "system" tests?

Coming from rspec, we can run our feature specs with Capybara pretty fast as long as we don't enable JavaScript. How can we speed up the start and/or execution time of the built-in Rails 5 system tests?

My current application_system_test_case.rb:

require 'test_helper'
require 'capybara/poltergeist'

class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
  driven_by :poltergeist
end