gwp
It can be programmed using its filesystem (9p) interface, and extended by adding plumber(4) rules.
A demo by Russ Cox: https://youtu.be/dP1xVpMPn8M
It's an intentional choice, that's why it compiles code so fast. Also because of that it's a lot simpler than say GCC. Before Go, the Plan 9 C compiler was designed in a similar manner too (I think the Go compiler was forked from it).
I think the simplicity aspect is even more important than the compiling speed. It's easier and cleaner to keep the compiler simple and write optimized assembly code by hand when it's needed. That way, the compiler doesn't get so messy (fewer bugs, easier to maintain...) and the written program is of better quality (humans can produce better code than compilers).
I think the real reason sites like Reddit want you to install their app is that they can't both have fancy JS and speed.
Web browsers are ridiculously complex and slow, and slow means bad UX. An app is much simpler (and thus faster) than a JS-heavy "web app" on Chromium/whatever (which is many millions of lines of code).
HN is fine with just a website and doesn't need an app because it doesn't have fancy JS.