I'm just now building a node app to filter point clouds, so lots of number crunching. In two days I've got something in javascript that's faster than the C++ solution I've been working on for a week. Mostly because javascript/node makes it trivial to parallelize file IO while doing work in the main thread. This app reads 13 million points from 1400 files (~200mb), filters all points within a clip region, and writes the 12 million resulting points in a 300MB file, all in 1.6 seconds. (File reads were cached by OS and/or disk due to repeated testing, but file writes probably not)
My personal conclussion is that javascript can rival or even exceed the performance of C++, not because it's inherently faster, it's obviously not, but because it makes it much easier to write fast code. For the highest possible performance you'll defenitely want to use C++ but sometimes you'll have to spend multiple times the work to get there.