Promise.all for parallel execution in Node.js - Performance & Optimization

Performance: Promise.all for parallel execution

MEDIUM IMPACT

This affects how quickly multiple asynchronous tasks complete and how the event loop handles concurrency.

Running multiple asynchronous tasks and waiting for all to finish

Node.js

async function fetchParallel() {
  const results = await Promise.all([fetch(url1), fetch(url2)]);
  return results;
}

Tasks run in parallel, reducing total wait time to the longest single task duration.

📈 Performance GainReduces total wait time, improves input responsiveness (INP)

Running multiple asynchronous tasks and waiting for all to finish

Node.js

async function fetchSequential() {
  const result1 = await fetch(url1);
  const result2 = await fetch(url2);
  return [result1, result2];
}

Tasks run one after another, increasing total wait time and blocking event loop longer.

📉 Performance CostBlocks event loop longer, increases total wait time linearly with number of tasks

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Sequential await calls	N/A	N/A	Blocks event loop longer	[X] Bad
Promise.all parallel execution	N/A	N/A	Non-blocking, faster completion	[OK] Good

Rendering Pipeline

Promise.all schedules multiple asynchronous operations concurrently, allowing the event loop to handle them without blocking. This reduces waiting time before the next paint or interaction can occur.

→Event Loop

→Task Scheduling

→Rendering

⚠️ BottleneckWaiting for all promises to resolve before continuing can delay next UI update if tasks are sequential.

Core Web Vital Affected

INP

This affects how quickly multiple asynchronous tasks complete and how the event loop handles concurrency.

Optimization Tips

1Use Promise.all to run multiple async tasks in parallel for faster completion.

2Avoid awaiting promises sequentially to prevent blocking the event loop longer.

3Parallel execution with Promise.all improves interaction responsiveness (INP).

Performance Quiz - 3 Questions

What is the main performance benefit of using Promise.all for multiple async tasks?

A Runs tasks in parallel, reducing total wait time

B Runs tasks sequentially to avoid race conditions

C Blocks the event loop until all tasks finish

D Increases bundle size by adding extra code

DevTools: Performance

How to check: Record a performance profile while running your async code. Look at the timeline for long blocking tasks and how async tasks overlap.

What to look for: Shorter total blocking time and overlapping async tasks indicate better parallel execution.