For what it's worth, the network I/O model design+implementation is rarely what makes the difference between high performance, or not, 'servers' -- and that is a mostly solved problem, at least in terms of patterns and practices that are adopted by 'modern' servers (few threads, no more than actual hardware CPU cores, async.multiplexing, etc).

High performance is about everything else, including processing actual incoming requests efficiently, reducing block/busy time in threads, caring a lot about memory access/caches. They key IMO is concurrency and migrating blocking operations to other dedicated threads, and this is where an efficient coroutines implementation would help both with accomplishing that task, but, perhaps more importantly, keeping the programming model simple.

C++17 will introduce support for reusmable functions ( http://blogs.msdn.com/b/vcblog/archive/2014/11/12/resumable-... ). This will be a game changer. Now, you can approximate this by e.g creating task abstractions and use lower-level(but slow) setjmp/longjmp functions, or just design tasks/functors that hold state that can schedule other tasks in turn, and so on. It works, but the overhead, mental, but also n terms of processing/executing/scheduling can be too great.