Last decade I wrote a server based on the old API and BindIoCompletionCallback. I can't comment on the performance differences, but the newer threadpool API is much better to work with.

IMO one of the biggest problems with the old API is that there is only one pool. This can cause crazy deadlocks due to dependencies between work items executing in the pool. The new API allows you to create isolated pools.

The old API did not give you the ability to "join" on its work items before shutting down. You could roll your own solution if you knew what you were doing, but more naive developers would get burned.

You also get much finer grained control over resources.

I'm really lucky to have decided to dig into this stuff (for PyParallel) around ~2011, by which stage Windows Vista (where all the new APIs had been introduced) had been out for like, 5-6 years, so no, I never had to deal with the old APIs, thankfully.

I really sympathize with Len Holgate though... he's had to nurse Server Framework through all of that transition, which must have been painful: http://www.serverframework.com/asynchronousevents/2016/06/an...