They load in the background. Look at the second video attempting to attack Slack. Look closely at the first tab in the top left corner, you can see that it is loading and eventually settles on Slack before the victim clicks the button. The attacker website has a delay on the click button to allow it to finish.
The exploit requires pages to load instantly. The first person was saying it usually takes a few hundred ms to load a page (at least). The second person points out that you can load the page in the background so it is in the local browser cache already, in which case loading is near instant.
I understood the first comment as tongue in cheek, because the web has become very slow. It's a legitimate argument, too, but I read it as at least a bit tongue in cheek.
How so? The page with the double-click prompt immediately changes the parent page behind it to the target location, and it can easily show a loading indicator for a couple seconds to wait for the target page to render before prompting the user to double-click.
