Before you check out this post I'd suggest you take a read of this earlier one, and the warnings it contains. What this posts talks about is a method that is far from official, and is likely not something anyone would recommend.
Once you've done that and feel that any risks are worth it - forge ahead and inject some into the New New (yep, the new new) Responsive Shopify Checkout. The checkout testing store has some injected CSS in place so you can get an idea of the loading delays and of course proof that it works!.
With luck, by the time you read this post the css has already been enabled and no further hackiness is required.
Now I don't waste time putting together super detailed "how to" posts so you'll need to do some learning on your own. I'm better left for cool branding, design or code things - not fancy documentation! I would assume there's enough typos and horrid grammar to scare most people so you'll need to pretend those things don't exist...
First Step: Make your checkout css file.
You can call it whatever you like, but for my checkout test store, I called mine injection.scss. It sounds exciting which is (to me!) important.
Step 2: Edit your Google Analytics settings
In the box that appears add the code found in the Gist below.
Be sure to change the scss filename to whatever you're using. That's it.