Under the hood

The next slide is already loaded

<script type="speculationrules">
  { "prerender": [{ "urls": ["/2/", "/4/"], "eagerness": "immediate" }] }
</script>

Both neighbours are prerendered, so the next slide is built and painted before it is asked for. Browsers without speculation rules get rel=prefetch instead, added by the runtime rather than the markup — so nothing fetches twice.