If your page starts with just a "loading" screen, then you may be in trouble. I don't think all search engines support javascript. Google states they do but Its best not to depend on it.
Also, your method requires users to make a minimum of two requests to the server just to receive some useful content. This will be a problem if your pages are visited very frequently because some users might feel like they may be stuck on the loading screen forever while the server is trying hard to fulfill the second request to other users.
To see what I mean visually, go to http://www.webpagetest.org/ and run your page through it and see how other web browsers load it.
If you really have to have some sort of loading indicator, try to do it without requiring another request, otherwise search engines will think your site is slow.
For example, make text on the screen that shows the word "loading" and use javascript to detect when time-consuming components like large images have been loaded. and make the word "loading" disappear after. At least that method will appear faster and people can still see content while other content is loading, and there will be one less server trip required.
Also, run your page with google's page-speed insights to see what they think as well.