Yes, I am currently writing a masters thesis about it! It is a "research language that [is] currently under heavy development" as the README says so I don't recommend using it for anything other than a research/toy project. But you would be surprised how well it works: Algebraic effects can give you 98% the power of monads while being much faster and easier to understand. And then Koka uses mimalloc [0], Perceus [1] and tail recursion modulo cons which make Koka programs almost as fast as C++. For example, in the benchmarksgame, Haskell takes 5x as much time on the binarytrees benchmark as the fastest implementation, while Koka is 50% slower (similar to other C/C++ and Rust implementations).

[0]: https://github.com/microsoft/mimalloc [1]: https://news.ycombinator.com/item?id=25464354