The first Fabric file-transfer benchmark managed about 169 MB/s. Buffer sizes, flow-control windows, storage and CPU all looked like reasonable suspects. None of them was the main problem.
The connection driver woke on a fixed five-millisecond timer. The function behind that timer did not merely check for a new stream; it drove reads, writes, flow-control updates and wake-ups for the entire multiplexer. Everything waited for a gate which opened 200 times per second.
Replacing the polling loop with the runtime’s actual wake-up mechanism moved the same in-memory benchmark from 168.9 MB/s to 628 MB/s. Peak memory fell as well because frames no longer piled up while waiting for the next tick.
Five milliseconds sounds tiny when looking at one sleep call. Put it in front of every useful piece of work and it becomes the fastest thing in the network.