So there’s really a lot. Each framework has it’s own ideas of how to manage threads. Perfect, Kitura, & Vapor manage their own, and are multi-threaded. Zewo is single threaded and meant to be run in parallel, more like Go, you just have to spin up as many processes as you have logical processors.
This gets a little more complicated, as Apple just announced the Server APIs workgroup, where they intend to implement and standardize the low-level items like HTTP Parsing, Sockets, etc… (see: https://swift.org/server-apis/). Once that happens, there may be a shift towards utilization of those APIs in general, especially given that some of the project leads are also the creators of Vapor, Zewo, & Kitura.