Not the craziest achievement, but while redesigning inboxes.app I managed to hit 99/100 on PageSpeed Insights on Desktop.
A lot of it comes down to caching, a simple website, using SVGs where possible and keeping external assets and plugins to a minimum.
I'll get that last 1% another time, but for a re-design I'm happy I was able to get a better looking website with an even better PageSpeed score 👌