Increasing Site Speed: Important Reminders

Most important, this tool is way better than Google's PageSpeed Insights.
1. Default settings throttle connection when testing. To remove throttling, change to Native Connection (No Traffic Shaping) in advanced settings.
Compress images before uploading.
1. Compresspng.com.
2. Compressjpeg.com.
3. WP Smush plugin (WordPress).
Turn on HTTP compression (GZIP).
1. Most hosting providers will do this automatically. If, for some reason, it's not being done, contact your hosting provider- it should take them a few seconds to enable.
2. Use GZIPwtf.com to test.
Caching.
1. HTML5BoilerPlate.
2. W3TotalCache (WordPress).
3. When analyzing browser caching, keep in mind the authentic change frequency of assets in order to avoid not receiving fresh content.
Combining CSS and JS.
1. Build tools (Grunt, Gulp, Webpack).
2. Don't use a WordPress plugin for this if on that platform - could be ineffective and slow down site.
Using a CDN is very important if traffic isn't local to your server.
1. As an SEO, it's very important to know where clients' servers are, if their traffic is non-local, and if they're using a CDN.
2. Solutions.
  1. Photon (WordPress) (Free).
  2. MaxCDN.
  3. Cloudflare (Free).
Upgrade to PHP7 (relevant apps).
Enable HTTP/2.
1. Use an SSL.
  1. Letsencrypt.com for free SSLs.
WordPress.com VIP (enterprise level) will automate a decent amount of this.