I've pasted that snippet and while it did indeed render fast, adding a newline anywhere caused the whole thing to hang for 12 seconds on my phone. Try it - you'll get a similar effect.

It's easy to just render a large page from scratch. Reflowing half of it by e.g. adding some vertical space is what's really hard.

EDIT: Actual numers are lower, but still troubling:

Parsing time: 771.7 milliseconds.

Browser time: 12078.1 milliseconds.