Calling tasks asynchronously in Flask - Performance & Optimization

Performance: Calling tasks asynchronously

HIGH IMPACT

This concept affects how quickly the main web request responds by offloading long tasks to run separately, improving user experience and server responsiveness.

Handling a long-running task during a web request

Flask

from flask import Flask, request
from threading import Thread
app = Flask(__name__)

def async_task():
    long_task()

@app.route('/process')
def process():
    Thread(target=async_task).start()
    return "Task started asynchronously"

The long task runs in a separate thread, allowing the request to respond immediately.

📈 Performance GainResponse is fast, reducing INP and improving user experience.

Handling a long-running task during a web request

Flask

from flask import Flask, request
app = Flask(__name__)

@app.route('/process')
def process():
    # Long task runs synchronously
    result = long_task()
    return f"Task done: {result}"

The long task blocks the request, delaying response and causing poor user experience.

📉 Performance CostBlocks rendering and response for the entire task duration, increasing INP significantly.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Synchronous long task	Minimal	0	Blocks response delaying paint	[X] Bad
Asynchronous task with thread	Minimal	0	Immediate response enables fast paint	[OK] Good

Rendering Pipeline

When tasks run synchronously, the browser waits for the server response, delaying rendering and interaction. Asynchronous tasks free the server to respond quickly, allowing the browser to paint and become interactive sooner.

→Server Processing

→Response Time

→Interaction Readiness

⚠️ BottleneckServer Processing blocks response when tasks run synchronously.

Core Web Vital Affected

INP

This concept affects how quickly the main web request responds by offloading long tasks to run separately, improving user experience and server responsiveness.

Optimization Tips

1Never run long tasks synchronously during a web request.

2Use threads or background workers to handle long tasks asynchronously.

3Asynchronous tasks improve interaction responsiveness (INP) by freeing the request.

Performance Quiz - 3 Questions

What is the main performance benefit of calling tasks asynchronously in Flask?

A The browser downloads assets quicker.

B The server uses more CPU for each request.

C The web request responds faster, improving user interaction.

D The database queries run faster.

DevTools: Performance

How to check: Record a performance profile while triggering the endpoint; look for long server processing blocking the main thread.

What to look for: Long gaps before first paint and interaction indicate synchronous blocking; short gaps indicate good async handling.