GitHub Pages runs everything through a Fastly CDN. You can tell like this:

  curl -i https://simonw.github.io/

I get this:

  HTTP/2 200 
  server: GitHub.com
  content-type: text/html; charset=utf-8
  permissions-policy: interest-cohort=()
  last-modified: Wed, 16 Nov 2022 21:38:29 GMT
  access-control-allow-origin: *
  etag: "63755855-299"
  expires: Wed, 23 Apr 2025 18:20:50 GMT
  cache-control: max-age=600
  x-proxy-cache: MISS
  x-github-request-id: 3D02:22250F:11BEDCA:123BE7A:68092D2A
  accept-ranges: bytes
  age: 0
  date: Wed, 23 Apr 2025 18:10:50 GMT
  via: 1.1 varnish
  x-served-by: cache-pao-kpao1770029-PAO
  x-cache: MISS
  x-cache-hits: 0
  x-timer: S1745431851.518299,VS0,VE110
  vary: Accept-Encoding
  x-fastly-request-id: 0df3187f050552dfde088fae8a6a83e0dde187f5
  content-length: 665

The x-fastly-request-id is the giveaway.