JavaScript is very fast if you want/need it to be. Emscripten/asm.js approach is in the same category as native for example. Your simple code won't be too fast, but not all code needs to be.

Also, have you heard of JavaOS? https://en.m.wikipedia.org/wiki/JavaOS

Define slow. JS is plenty fast for a dynamic language.

Mostly because Linus won't accept anything other than C on his beloved kernel.

Sun had experimental support for Java drivers on Solaris, Android has support for writing drivers in Java, Android Things only allowed for Java written drivers.

JavaScript isn't slow