It's complete reimplementation of Express, with support for all APIs, except built on top of uWebSockets. It's the only library that fully implements Express' API while using uWS.
It's about 5 times faster than Express. Please check Github for more benchmark info. I have started replacing Express with this library in my projects and performance improved by 1.5-4 times depending on route. In 1 case file serving throughput increased from 230 MB/sec to 800 MB/sec.
Please keep in mind that it's a very new library and there could be bugs. Always test your application after switching!