There are many configuration options with inpact on performance in Roundcube and behind it (Database and Webserver). As Roundcube installation administrator you can set them to some defaults and prevent users to change them by using the ‘dont_override‘ option.
I tested some options and created my tiny boost list.
Here’s the list of options with impact on performance
Messages listing is a main task of a mail client. Sorting is expensive. First of all you should use an IMAP server with SORT capability. If message list displaying is still too slow you should set ‘message_sort_col’ to an empty string.
To keep the IMAP session open, Roundcube is sending a request to the IMAP server every ‘keep_alive’ seconds. When a user has a mail task window open, Roundcube will also check for new mail using this interval. To decrease the load, don’t set this too low.
This really slows things down. With ‘skip_deleted’ enabled, Roundcube needs to check the status of each message when displaying the list, count the messages, etc. E.g.
More performance = false
With enabled caching, Roundcube stores message counts, folder list and message headers in the database. You should consider enabling this if access to your database server is faster than access to the IMAP server.
If, with enabled caching, your database is growing too fast, you can lower the lifetime to not store old messages. About 5-7 days.
Optimize your Mail Domain Setup:
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cat /etc/hosts 127.0.0.1 localhost.localdomain www.mydomain.com # should be: 127.0.0.1 localhost.localdomain www.mydomain.com mail.mydomain.com