Some plugins set a cookie to identify sessions. This happens because visitors sometimes need to see unique content, such as the contents of a shopping cart. When such a cookie is set, our Varnish cache no longer works. This can result in a slower website. To find out if your website is using a session, you can do the following:
- In Chrome, start the Developer Tools (CTRL+SHIFT+I).
- Choose the Network tab in the Developer Tools
- Reload your page with CTRL+F5
- Click on the first request (1), click on Cookies (2) and see if you see a cookie with PHPSESSID (3), or something similar.
To find out which plugin sets the cookie, the only way is to disable all your plugins one by one and repeat the steps above. If the cookie disappears after you disable a specific plugin, you will know which plugin the cookie is setting.