The performance comes from using https://github.com/valyala/fasthttp instead of the stdlib net/http. From this project's FAQ:
>Why creating yet another http package instead of optimizing net/http?
Because net/http API limits many optimization opportunities.
And then you got your database and you speed is comparable to the rest of the world :)
What I think could work is a pure in memory database with server combination. Something like redis for websites.
There are a plenty of in-process 'databases' for Go starting from a simple built-in map and ending with something like https://github.com/boltdb/bolt or even https://github.com/hashicorp/go-memdb .