How do you diagnose and troubleshoot issues with slow website performance?

The diagnosis of a slow website is carried out in stages:

1. Information gathering and symptom identification:

   • Use online speed testing tools (WebPageTest, GTmetrix, Google PageSpeed Insights).
   • Observe the site's behavior across different browsers and devices.
   • Analyze web server logs (Apache, Nginx) for errors and anomalies.
   • Survey users about specific scenarios where slowness occurs.

2. Frontend performance analysis:

   • Use browser developer tools (Chrome DevTools, Firefox Developer Edition) to analyze resource loading.
   • Evaluate load times for HTML, CSS, JavaScript, images, and fonts.
   • Look for blocking render resources and unoptimized scripts.

3. Backend performance analysis:

   • Check CPU, memory, and disk subsystem loads.
   • Use monitoring tools (Prometheus, Grafana, Nagios) to track metrics.
   • Analyze database query execution times.
   • Check application code performance (profilers, tracing).

4. Network infrastructure check:

   • Use ping and traceroute to check latency and packet loss to the server.
   • Check network interface load on the server and intermediate nodes.
   • Evaluate CDN performance if used.

5. Server configuration analysis:

   • Check web server settings (keep-alive, gzip, caching).
   • Optimize database settings (indexes, query caching).
   • Ensure sufficient resources (CPU, RAM) for current load.

After identifying the bottleneck, proceed to solve the problem. Example solutions include:

• Frontend:
  • Minify and compress static resources.
  • Lazy load images.
  • Optimize images (compression, modern formats like WebP).
  • Defer JavaScript loading (async, defer).
  • Use CDN for static resources.
  • Optimize CSS (critical CSS, remove unused styles).
• Backend:
  • Optimize database queries.
  • Cache data (Redis, Memcached).
  • Optimize application code.
  • Scale the server (vertical or horizontal scaling).
  • Use more powerful hardware.
• Network infrastructure:
  • Switch to a faster connection.
  • Optimize routing.
  • Configure CDN.
• Server configuration:
  • Fine-tune web server and database settings.
  • Increase resource limits.

Example of using tools:

# Check site availability and response time
curl -s -w "%{time_total}\n" -o /dev/null http://yourwebsite.com

# Trace route to the server
traceroute yourwebsite.com

# Get performance metrics using Prometheus
# Assuming exporter is already installed and configured
curl http://localhost:9090/api/v1/query?query=node_cpu_usage_seconds_total

When working with a database:

-- Analyze the performance of a specific query in PostgreSQL
EXPLAIN ANALYZE SELECT * FROM users WHERE status = 'active';

-- Find slow queries in MySQL
SHOW PROCESSLIST;
-- Or analyze the slow query log if enabled

After applying changes, always retest to ensure the issue is resolved and no side effects occur. Use monitoring for continuous performance tracking.