For writing high-performance JavaScript, I'm really hoping that AssemblyScript takes off[1]. The project is still a bit in early stages, but theoretically, it can have the same role as Cython: you write JS/TS code, find the slow parts, and get perf improvements by adding stricter types to your code and changing your code to not use dynamic JS features. You can stay pretty much in JS rather than having to switch to a completely new language to get predictable fast performance.

[1] https://github.com/AssemblyScript/assemblyscript