I imagine it's because this is a web jupyter book or something. It's definitely extremely slow loading the page on mobile even after downloading the assets, so it's probably super unoptimized.

Right, we didn’t get around to splitting the js bundles up yet but will do so soon, thanks for the reminder. Currently the main bundle is the same for the page and the editor which you can try at https://nextjournal.com/try

Heavy page weights really should be called out in the submission title, IMHO. Not all of us are on unlimited data plans.