I am hosting a number of websites on the Pantheon platform. A few of my sites are not caching properly because some of the plugins are using $_SESSIONS and setting a cookie SSESS or SESS in the header. This is resulting in very slow TTFB (up to 5000ms in some cases) and therefore decreasing performance of my sites significantly.
There is no way to update the varnish cache settings to cache these cookies on the pantheon platform I am told.
Is this a varnish cache limitation? Is it bad practice from the plugin developers to use this way of accessing and not caching session information?
The only suggestion I was given by pantheon is to disable these plugins (there are several across my sites) which isn't very helpful because they are required. I have also reached out to the plugin developers to see if they can assist but I was told that this is a standard way of accessing PHP session information.
More info from Pantheon here: https://pantheon.io/docs/wordpress-sessions#varnish-or-caching-is-not-working-when-a-plugin-or-theme-that-uses-code-classlanguage-text_sessionscode-is-enabled