https://world.hey.com/lewis/faster-rails-system-tests-f01df53b

session_system_test_helper.rb

module SessionSystemTestHelper
  def sign_in_as(user)
    visit root_url # any fast-to-load page will do

    cookie_jar = ActionDispatch::TestRequest.create.cookie_jar
    cookie_jar.signed[:session_token] = user.sessions.create.token
    page.driver.browser.manage.add_cookie(name: "session_token", value: cookie_jar[:session_token], sameSite: :Lax, httpOnly: true)
  end
end

For the initial page load you can use a blank html page, so it's as fast as possible.

Something like visit root_url + "blank.html" where blank.html is a file served from /public.