Achieving great site speed is not just a coincidence, as there are lots of efforts being put in to get there. Choosing the right tech tools for the job, optimizing the code for performance, and maintaining it all besides scaling up, all require a lot of competency and planning.
For me, the fastest sites are Google, Twitter, and DEV.
Regarding to DEV, I would recommend reading these blog posts by the co-founder @ben, explaining behind the scenes stuff:
What it Takes to Render a Complex Web App in Milliseconds
Ben Halpern ・ Aug 18 '17 ・ 4 min read
What are the fastest sites you visit daily?