Toolset:

• Google PageSpeed Insights
• WebPagetest.org
• Pingdom Website Speed Test
• YSlow browser extension

WordPress Performance Issues

• P3 (Plugin Performance Profiler)
• NewRelic

Keep website lean

• Delete All Comments
• WP DBManager
• WP-Optimize

Minify everything

• Better WordPress Minify
• Autoptimize

Image Compression

• WP Smush.it
• Bulk Resize Media
• Kraken Image Optimizer
• EWWW Image Optimizer

Caching

• WP Super Cache
• W3 Total Cache
• WP Rocket

You can simulate load and measure performance on your site with tools like:

• ab Apache HTTP server benchmarking tool
• blitz.io
• loader.io

or  simple test using ab  like:

$ ab -n 100 -c 5 http://example.com/

Use CDN

• MaxCDN
• CloudFlare
• Amazon CloudFront