Turing completeness says absolute nothing about performance. If my embedded logic programming library takes 2ms to run a query and yours takes an hour - I can actually put mine to useful work.

Granted many languages simply move this kind of work to external C dependencies and such. However this doesn't address the other kinds of language extension such as idiomatic ad-hoc type systems and predicate dispatch I mentioned.

Why shouldn't our programming languages capture these heterogenous use cases comfortably?