If you do want to write javascript however, why not write something more application based, why does it need to be raw linear algebra? Like an image processing library, or a linear-transforms-for-svg library[0]? Why do you need the raw linear algebra primitives, such as being able to map across elements?
Your snark towards my criticism is also absolutely ridiculous - no offense to the author, I think this is quite cool, but it is in no way "forward thinking". I could link to tens of other javascript linear algebra libraries, so why is this one the best? And more importantly - why should it be shielded from criticism?
[0] I will admit, there is something similar to this, based off this library: https://github.com/mateogianolio/vectorious-plot which slightly detracts from my argument, but also bolsters it - as it proves that this library is only really useful with an application library wrapped around it.
Optimizations, using Typed Arrays, could make the code considerably faster. C++ addon code could make it faster still. Intrinsics within that addon code could use SIMD. While this library certainly is a lot slower than the equivalent C or Fortran, it's got potential, and it's not something that I'd dismiss as being pointless because it's a particular thing that's written in JavaScript.
Also, in the browser, language choices are very limited. Another reason why having JavaScript code that does a variety of things (including vector maths) is useful.