Misc perf improvements

[tunetheweb]

Inspired by @rviscomi 's work on the main HTTP Archive site in HTTPArchive/httparchive.org#796 I've had a look at this site

We're just missing INP target and also have load issues (likely when content is not cached in CDN as quiet time between issues):

Checking histrorical views it looks like the INP has been better historically so this may be a once off, but would still be nice to get some head room.

Our articles are quite big, especially some of them, so while we've been careful with our JS, we could still look at what other layout improvements we could make.

I've made the following changes:

• Changed use of innerText to textContent to prevent reflow as innerText is aware of CSS so requires a style recalc.
• Make the "Show Descriptions" buttons below each text visibility: hidden rather than display: none to prevent shifts/layouts when the buttons are activated by JS. Also add a no js full hide to avoid the gap if buttons aren't activated.
• Implement speculation rules with moderate eagerness to improve load times when navigating between chapters. Should also help INP if we can get some of that load layout and JS execution out of the way earlier.
• Add LoAF monitoring for future monitoring - note I've only set these up with the production params with no origin trail token since it's going live soon.

I was not able to verify any noticeable improvement in Perf tracing or Lighthouse as these really are micro-optimisations and there's a lot of variability but still worth doing IMHO.