Isn't Japronto a Python micro framework?
Caddy for Go handles http/1, http/2, and http/3 and will automatically obtain TLS certs for your site. Cloudfare has had a million open sockets on Go which would fulfill your million requests Japronto has had.
If you know Python, then use Japronto which states on Github there is no new development happening and has been dormant since 2020. If you know Pascal, then use Brook which hasn't been updated in a year.
With your C# thread it seems like you're just starting and wondering what is best to use? But a lot depends on what you intend to do. Windows, Linux, Mac? Desktop apps or Web apps? Do you intend to do mobile apps?