Wordpress website is loading slow: slow admin-ajax.php and high TTFB

Symptoms

• Wordpress website works slowly.

• Some sections of the website return 404 error.

• https://example.com/wp-admin/admin-ajax.php also works slowly.

• High TTFB can be observed:

  # # curl -s -w '\nLookup time:\t\t%{time_namelookup}\nConnect time:\t\t%{time_connect}\nSSL handshake time:\t%{time_appconnect}\nPre-Transfer time:\t%{time_pretransfer}\nRedirect time:\t\t%{time_redirect}\nStart transfer time:\t%{time_starttransfer}\n\nTotal time:\t\t%{time_total}\n' -o /dev/null http://example.com
  Lookup time: 0.055
  Connect time: 0.224
  SSL handshake time: 0.000
  Pre-Transfer time: 0.225
  Redirect time: 0.000
  Start Transfer time: 1.395
  Total time: 0.395

  Note: A high value can be considered when the substraction of "Start Transfer" value from the "Pre-Transfer" value is more than 1 second.

Cause

Some WordPress plugins affect the website performance.

Resolution

1. Log in to Plesk.

2. Disable plugins (for example WP-SpamShield) one by one to find the cause in Domains > example.com > WordPress > Plugins > Deactivate button, or completely remove the Plugin from the Trash icon: