I wrote the details of this up here.

It relates to this tweet, which some of you may have seen.

To summarise:

• We could propose a change to libuv or float some patches on top of it which removes a ~12µs delay when we poll the event loop and there is no pending io
• This results in 25x (~2m ticks versus ~80k ticks per second) better throughput for setImmediate in the microbench provided in the gist above and on the original tweet from Jarred Sumner

Am happy to work on a PR for node.js or libuv if folks think this is worth doing and/or likely to land.

Haven't worked out the actual logic for when to set this flag yet so would likely need some feedback on that from the libuv maintainers.