This is most likely because allocating typed arrays is really, really slow in JavaScript. IIRC it has to do with the fact that there is a lot of flexibility regarding backing buffers or something; each typed array object has about 200 bytes of overhead compared to a dozen bytes for a plain array or object (roughly).

Typed arrays are mainly faster in scenarios where you allocate one or a few large TypedArrays once and then re-use them a lot.

That also explains your experience with many small arrays. That's basically the worst way to use TypedArrays.