The real killer feature is async. Since a modern web request typically spend most of the time waiting for database calls, file system requests or similar, a naively coded server in most languages can handle relatively few requests per thread, so you scale up the number of threads to something like 100 per core, and now the overhead of running and switching between these treads is limiting the performance.

Being used to Node, I was flabbergasted when writing C for Linux* . The file system commands just leave my thread hanging while the result is being generated, if I use it on a network drive it might hang for a minute before timing out, so I have to make a tread for each file system command, solely so that it can stall without bringing down the whole application.

* I have no delusions that Windows is any better, Linux is just what I have first hand experience with.