Lazy loading vs eager loading in Flask - Performance Comparison

Performance: Lazy loading vs eager loading

MEDIUM IMPACT

This concept affects how quickly data is fetched and rendered on the page, impacting initial load time and responsiveness.

Fetching related database records in a Flask app using SQLAlchemy

Flask

from sqlalchemy.orm import joinedload
posts = Post.query.options(joinedload(Post.author)).all()
for post in posts:
    print(post.author.name)

This loads posts and their authors in a single query, reducing database calls and speeding up rendering.

📈 Performance Gainsingle database query regardless of number of posts, reducing load time significantly

Fetching related database records in a Flask app using SQLAlchemy

Flask

posts = Post.query.all()
for post in posts:
    print(post.author.name)

This triggers a separate database query for each post's author (N+1 query problem), slowing down page load.

📉 Performance Costtriggers N+1 database queries, increasing load time linearly with number of posts

Performance Comparison

Pattern	Database Queries	Initial Load Time	Network Overhead	Verdict
Lazy Loading	Multiple (N+1 queries)	Lower if few accessed	Lower initially, higher if accessed later	[!] OK
Eager Loading	Single batched query	Higher upfront	Higher upfront, lower later	[OK] Good

Rendering Pipeline

Lazy loading delays fetching related data until it is accessed, reducing initial data processing. Eager loading fetches all related data upfront, increasing initial processing but reducing later delays.

→Data Fetching

→Template Rendering

→Network Requests

⚠️ BottleneckData Fetching stage due to multiple database queries in lazy loading

Core Web Vital Affected

LCP

This concept affects how quickly data is fetched and rendered on the page, impacting initial load time and responsiveness.

Optimization Tips

1Avoid lazy loading if it causes many database queries on page load.

2Use eager loading to fetch related data in one query for faster initial rendering.

3Balance eager and lazy loading based on data size and user interaction patterns.

Performance Quiz - 3 Questions

What is the main performance problem with lazy loading in database queries?

A It loads all data upfront causing slow start

B It blocks rendering completely

C It causes many small queries increasing load time

D It increases bundle size

DevTools: Network

How to check: Open DevTools > Network tab, reload page, and observe number of database API calls or data fetch requests.

What to look for: Multiple small requests indicate lazy loading with many queries; a single larger request indicates eager loading.