In part 1, we built the core stack/microtask/macrotask model. In part 2, we saw how the browser and Node implement that model differently — rendering interleaved with tasks in the browser, libuv's phase-based loop in Node. Both of those posts share an unspoken assumption: your callbacks are fast. A few milliseconds, do their thing, return, let the loop move on. But what happens when that assumption breaks? What happens when you genuinely need to do slow, CPU-bound work — and there's no I/O to...