Acme: http://acme.cat-v.org/
It can do everything Emacs can but it's so lightweight it almost doesn't exist. Its power comes from the fact that it brings your external tools together.
It can be programmed using its filesystem (9p) interface, and extended by adding plumber(4) rules.
> I was always under the impression that Go never had a great optimizing compiler. It was never a primary focus given the limited developer resources.
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).
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