> Doesnt fast code on these types fly [in] the face of julia static optimization ethos?

What does that even mean? Julia's union types aren't intentionally slow, they just aren't implemented very efficiently yet. Major revisions of how they're implemented are definitely on the roadmap, not far away.

For fine-grained parallelism of the type you'd use MPI for, 1ms overhead (assuming that's pure overhead above and beyond the actual cost of data movement) could be significant, sure. If you have calculations that need to go for thousands of individually cheap iterations, it adds up.