When we were finalizing features for Expressive 3, we had a number of users testing using asynchronous PHP web servers. As a result, we made a number of changes in the last few iterations to ensure that Expressive will work well under these paradigms.
Specifically, we made changes to how response prototypes are injected into services.
The article starts by talking about the issue with the previous response prototype handling, mostly that, in an async world, all responses would be sharing the same instance rather than rebuilding a new one for each request. It then explains their reasoning behind even worrying about async support in the framework's response handling noting that the major reason had to do with the performance gain. The post mentions the Swoole extension and shows how to install it via PECL but you'll need to check out the project's documentation to see how to create a server that makes use of it.