This can be caused by many different things, the default configuration of the perfects etup is very fast, but there might be changes in your server configuration that slow the system down or high load etc. So to find the bottleneck, you have to check services step by step.
As a first step, i would install another webmail client beside oundcube e.g. squirrelmail. Squirremail is quite fast, so if you dont get the same delay in squirrelmail that you get in roundcube, you have narrowed down the issue to roundcube. If you experience the same delays, then it might be related to the imap server configuration or harddisk performance / load of the server.