What is the most impactful optimization for API response times?

Caching at the right layer provides the biggest improvement. A CDN cache hit returns responses in under 10ms from an edge server without touching your origin. A Redis cache hit returns in 1-2ms versus 50-500ms for a database query. Start by adding Cache-Control headers for public data so CDNs and browsers cache responses, then add Redis caching for expensive database queries. Only after caching is optimized should you focus on query optimization, compression, and architectural changes.

When should I use Redis caching vs HTTP cache headers?

Use HTTP cache headers (Cache-Control, ETag) when responses can be cached by CDNs and browsers, the data is not user-specific or can be marked as private, and you want to reduce load on your entire infrastructure including the application server. Use Redis caching when the response depends on expensive server-side computation, you need fine-grained cache invalidation, the data combines multiple database queries, or you need to cache data that cannot be cached by HTTP intermediaries due to varying parameters.

How do I avoid N+1 query problems in my API?

N+1 queries occur when you fetch a list of items then make a separate query for each item related data. Fix this by using JOIN queries to fetch related data in a single query, using IN clauses to batch-fetch related records by ID, using ORM features like eager loading (Sequelize include, Prisma include, ActiveRecord includes), or using DataLoader for GraphQL which batches and deduplicates queries within a single request. Monitor your query count per request to detect N+1 issues automatically.

Should I compress API responses at the application level or proxy level?

Compress at the proxy level (Nginx, CDN) whenever possible. Proxy-level compression is more CPU-efficient because Nginx is optimized for it and can cache compressed responses. Application-level compression (Express compression middleware) uses Node.js CPU which is better spent on business logic. Configure Nginx with gzip_types and brotli_types for JSON and text content types. Only use application-level compression when you do not have a reverse proxy or need response-specific compression decisions.

When should I use async processing instead of synchronous API responses?

Use async processing (message queues, background jobs) when the operation takes more than 1-2 seconds, the result is not needed immediately (report generation, email sending, image processing), the operation can be retried on failure, or the operation involves external services with unpredictable latency. Return HTTP 202 Accepted with a job ID and poll URL. Clients check the job status periodically or receive a webhook when processing completes. This keeps API response times consistently fast regardless of backend processing time.

Optimizing API Response Times for Heavy Traffic