Lacking an event loop is not the biggest problem. Both libraries have a lot of allocation and other abstractions that get in the way of performance. Mio allow

Lacking an event loop is not the biggest problem. Both libraries have a lot of allocation and other abstractions that get in the way of performance. Mio allows you to work with the sockets directly and does not allocate (and its APIs are designed to be usable without allocating), making things much easier to reason about.