I appears that this is using HttpListener to implement the HTTP protocol:

https://github.com/sisk-http/core/blob/main/src/Http/HttpSer...

HttpListener, while not deprecated, is frozen and not taking new features. For example, it does not support HTTP 2. It might not be as high performance as Kestrel (the default web server in ASP.NET Core).

https://github.com/dotnet/runtime/issues/63941#issuecomment-...

So the authors of the framework might want to consider longer term whether layering onto of Kestrel or some other HTTP server implementation may make more sense.