Creating documents in Express - Performance Optimization Steps

Performance: Creating documents

MEDIUM IMPACT

This affects server response time and how quickly the client receives the document content.

Generating and sending a document response to the client

Express

app.get('/doc', async (req, res) => {
  const doc = await generateLargeDocumentAsync();
  res.send(doc);
});

Asynchronous generation frees the event loop, allowing other requests to be handled concurrently.

📈 Performance GainReduces server blocking, improves response time and throughput under load

Generating and sending a document response to the client

Express

app.get('/doc', (req, res) => {
  const doc = generateLargeDocumentSync();
  res.send(doc);
});

Synchronous document generation blocks the event loop, delaying response and increasing server load.

📉 Performance CostBlocks server event loop, increasing response time by hundreds of milliseconds under load

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Synchronous document creation	N/A (server-side)	N/A	N/A	[✗] Bad
Asynchronous document creation	N/A (server-side)	N/A	N/A	[✓] Good

Rendering Pipeline

Express creates the document on the server, then sends it over the network to the browser, which parses and renders it.

→Server Processing

→Network Transfer

→Browser Parsing

→Browser Rendering

⚠️ BottleneckServer Processing when document creation is slow or blocking

Core Web Vital Affected

LCP

This affects server response time and how quickly the client receives the document content.

Optimization Tips

1Avoid synchronous document generation to prevent blocking the server event loop.

2Use asynchronous or streaming methods to create documents efficiently.

3Monitor server response times to ensure fast document delivery and better LCP.

Performance Quiz - 3 Questions

What is the main performance issue with synchronous document creation in Express?

A It increases client-side rendering time

B It causes layout shifts in the browser

C It blocks the server event loop, delaying all requests

D It reduces network bandwidth

DevTools: Network

How to check: Open DevTools, go to Network tab, reload the page, and check the Time and TTFB (Time to First Byte) for the document request.

What to look for: Long TTFB indicates slow server document creation; shorter TTFB means faster server response.