Rails post-render flash succeed

I'm learning rails right now. Today for some odd reasons I put the line of flash after the redirect, which is obviously a bad practice. But strangely, after the redirect the flash works.

Isn't flash supposed to be used to carry the message to the next request?

Bearing this in mind I added a line that is supposed to makes the program stop for 10 seconds after the redirect. Then I noticed that the entire website will hold for 10 seconds before redirect. Why did redirect_to method wait 10 seconds? This will not be the case if I replace the redirect_to method with render. I've put the code blocks below.

redirect_to root_url
message = "Account not activated. "
message += "Check your email for the activation link."
flash[:danger] = message #can still see the flash after redirect

Entire Controller:

class SessionsController < ApplicationController
  def new
  end

  def create
    user = User.find_by(email: params[:session][:email].downcase)
    if user && user.authenticate(params[:session][:password])
      if user.activated?
        log_in user
        params[:session][:remember_me] == '1' ? remember(user) : forget(user)
        redirect_back_or user 
      else 
        redirect_to root_url
        sleep 10
        message = "Account not activated. "
        message += "Check your email for the activation link."
        flash[:danger] = message       
      end
    else
      flash.now[:danger] = "Invalid email/password combination"
      render "new"
    end
  end

  def destroy
    log_out if logged_in?
    redirect_to root_url
  end
end

Before Rails does anything with your controller method it reads and builds the response.

If you were to return immediately after calling redirect_to your flash wouldn't be captured and inserted into the appropriate methods.