The core WordPress files, plugins, and default theme are pretty well built and optimized. Slowness is generally caused by a slow theme or poorly coded plugin.
To diagnose which theme or plugin is causing the delay:
- Temporarily change to the default WordPress theme. If this resolves the loading speed problems, the issue is likely caused by the theme. If the issue is not resolved, restore the custom theme and continue testing.
- Disable all plugins.
- This should resolve the site performance issue. If the issue persists, restore your plugin settings, disregard the remainder of these instructions, and seek further assistance.
- Enable one plugin, then review the sites performance. If the site continues to work well, the activated plugin is not likely the source of the issue.
- Continue activating plugins one at a time, and testing after each, until the slowness returns. The most recently activated plugin is the culprit and should remain disabled until fixed or optimized.
Tips and tricks
- Check for updates. Newer versions typically contain bug fixes, one of which might fix what makes your site slow.
- Caching plugins will probably interfere with this testing. Test them last.
- Security oriented plugins, such as firewalls and login restrictors are known to be very resource intensive and are usually worth checking first. If such a plugin is the problem, try using CloudFlare's free service instead.
- If the site is getting too much traffic and the traffic is impacting your ability to test, edit your .htaccess file to prevent all web traffic other than yourself.