In this article I discuss why F# Async is a good thing for writing concurrent software on .NET and show how to implement your own Async specialized for low-concurrency use. As a sample application, I look at a simple CML-style blocking channel. 30-50 lines of custom async and threadpool implementation increase the throughput from 100-400 K to 1M messages a second. Concurrency? Again? It is hard to believe that after so many man-years of computer science research anyone would still ha[...]