Site works slowly. What can be checked?

Note: To get an idea what is a good value, compare your website performance against big service providers such as Google, Youtube, Amazon, Microsoft, etc.

Check Time To First Byte (TTFB) value for static (e.g. .html files) and dynamic (e.g. .php files) content:

# 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: 0.395

Total time: 0.395

Subtract the "Start Transfer" value from the "Pre-Transfer" value to get TTFB value. In the example above, the TTFB value is a lot less than 1 second and this means that the web server is not causing the issue. If the value is more than one second, follow the steps from this article to troubleshoot the web server further: TTFB is too high. Another reason can be due to exceeded FcgidMaxProcesses processes or reached pm.max_children PHP-FPM limit. Also, basic checking for possible DDoS attack can be performed.

Plugins, Themes periodically are updated and require higher PHP version. Switch PHP to highest possible version in Domains > example.com > Hosting Settings

The slow query log can be used to find queries that take a long time to execute and are therefore candidates for optimization. Activate MySQL slow query log and make sure that there are no entries. Contact website developer if slow queries are found. The article: MySQL performance is slow. How to improve it? - can help to investigate MySQL performance issue.

Try to open the website from a workstation in the other physical location (webpagetest.org or tools.pingdom.com). If from the other location/network it opens fine, contact system administrator to test network connection bandwidth (iperf) and stability (mtr). If the bandwidth is small, consider compressing website static content (images) – see the next step.

If most of bandwidth is taken by DNS, test website with another DNS servers, for example use 8.8.8.8 Google DNS.

• If the website is "heavy", optimize its size by compressing images, for example.

• Use Fiddler or webpagetest.org to find out which HTTP requests take the most time to execute: it can be slow authentication requests, .aspx pages compiled slowly due to antivirus or not optimized script.

• If the website is written in PHP and served by PHP-FPM, slowlog can be enabled. It allows logging scripts (not just their names, but their PHP backtraces too, using ptrace and similar things to read remote process' execute_data) that are executed unusually slow.

  To enable it go to Plesk > Domains > example.com > PHP Settings > Additional directives > Additional configuration directives

  CONFIG_TEXT: [php-fpm-pool-settings]
  slowlog = /var/www/vhosts/example.com/slow.log
  request_slowlog_timeout = 5s

  You can replace 5s with any other value. This will help find scripts which execute slowly. Image resizing function, network I/O related functions are some examples which will frequently show-up in PHP slowlog.

• To analyze the code for optimization (decrease execution time for scripts) contact the website developer. The optimized code is handled by web browsers faster.

• To reduce the size of transmitted data GZip compression can be enabled