It is relatively slow, but not as bad as you might think for real world usage. Sure, it'll be a *ton* slower than doing the same thing on an in-memory array with numpy. But for out-of-memory arrays the limiting factor is I/O, not CPU.

That said, yes, I have considered Cython, numpexpr, numba, etc. as being relevant here. It's just further down the priority list because the bottlenecks are currently elsewhere.